event lifecycle

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diff --git a/src/lua_event_bridge.zig b/src/lua_event_bridge.zig
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+//! Lua bridge for the extension UI event system (plan §7.6).
+//!
+//! This module bridges the native event machinery in `tui_event.zig` to
+//! Lua: it lets a Lua `panto.ext.on(name, handler)` callback participate in
+//! the SAME `EventBus` the native side fires, receive a bridged `event`
+//! object (`getComponent`/`setComponent` + payload fields), and either
+//! pass through a native default component, wrap it, or install a
+//! brand-new component DEFINED IN LUA.
+//!
+//! It is owned by the long-lived `LuaRuntime` (one `lua_State` for the
+//! whole session) and shares that state. It never opens its own state.
+//!
+//! ## Synchronous rendering (NOT the coroutine scheduler)
+//!
+//! A Lua-defined component's `render(width)` is on the engine's hot path:
+//! the differential renderer calls it and needs the produced lines back
+//! IMMEDIATELY. So the bridged vtable calls into Lua **synchronously** via
+//! `lua_pcallk` (the same model as `LuaRuntime.runCommand`), NOT through
+//! the libuv coroutine scheduler used for async tool batches. A render
+//! callback may not yield; if a Lua component blocks, that is a bug in the
+//! extension, and we surface a safe fallback line rather than hanging or
+//! corrupting the frame.
+//!
+//! ## Cache-derived `firstLineChanged`
+//!
+//! Each bridged component owns a native `RenderCache` (the same one native
+//! components use). The Lua side only returns an array of line strings per
+//! `render`; the bridge calls `cache.store(lines)` and derives
+//! `firstLineChanged` from the cache diff exactly like a native component.
+//! This keeps the cache-derived dirty-model invariant (§3.3) and frees Lua
+//! authors from implementing `firstLineChanged` correctly. A Lua-provided
+//! `firstLineChanged` override could be added later; cache-derived is the
+//! correct, safe default.
+//!
+//! ## Lifetime / ownership (§6, §7.4)
+//!
+//! Bridged components and Lua handlers reference Lua functions/tables that
+//! must survive as long as the engine borrows them. Both are `luaL_ref`'d
+//! into the registry; the refs are owned by this bridge and released at
+//! runtime teardown. A bridged component (its `luaL_ref` + `RenderCache`)
+//! is heap-allocated and tracked in `components`, freed on `invalidate`
+//! and at teardown. There is no "active component": each `emit` that
+//! produces a Lua component mints a fresh instance keyed by that
+//! boundary's own event, exactly like native components.
+
+const std = @import("std");
+const lua_bridge = @import("lua_bridge.zig");
+const ui_event = @import("tui_event.zig");
+const component = @import("tui_component.zig");
+const components = @import("tui_components.zig");
+const theme = @import("tui_theme.zig");
+const engine = @import("tui_engine.zig");
+
+const c = lua_bridge.c;
+const Component = component.Component;
+const RenderCache = component.RenderCache;
+const Event = ui_event.Event;
+const Payload = ui_event.Payload;
+const EventBus = ui_event.EventBus;
+const Handler = ui_event.Handler;
+
+// Metatable names (registered via `luaL_newmetatable`, looked up by
+// `luaL_checkudata`/`luaL_testudata`). The `event` userdata identifies a
+// bridged event object; the `component` userdata is a native-component
+// passthrough handle.
+const eventMtName = "panto.event";
+const nativeCompMtName = "panto.component";
+
+/// A component DEFINED IN LUA, bridged to the native `Component` vtable.
+///
+/// `table_ref` is a `luaL_ref` to the Lua table implementing
+/// `render(self, width) -> {strings}` plus optional `firstLineChanged` /
+/// `handleInput` / `invalidate` methods. The bridge owns a `RenderCache`
+/// and derives the dirty signal from it.
+pub const BridgedComponent = struct {
+    bridge: *EventBridge,
+    /// `luaL_ref` to the Lua component table.
+    table_ref: c_int,
+    cache: RenderCache,
+    /// True once freed, to make double-free / use-after-free a no-op.
+    dead: bool = false,
+
+    fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
+        _ = alloc;
+        const self: *BridgedComponent = @ptrCast(@alignCast(ptr));
+        return self.bridge.renderBridged(self, width);
+    }
+
+    fn firstLineChangedImpl(ptr: *anyopaque) ?usize {
+        const self: *BridgedComponent = @ptrCast(@alignCast(ptr));
+        return self.cache.firstLineChanged();
+    }
+
+    fn invalidateImpl(ptr: *anyopaque) void {
+        const self: *BridgedComponent = @ptrCast(@alignCast(ptr));
+        self.cache.invalidate();
+    }
+
+    fn handleInputImpl(ptr: *anyopaque, data: []const u8) void {
+        const self: *BridgedComponent = @ptrCast(@alignCast(ptr));
+        self.bridge.handleInputBridged(self, data);
+    }
+
+    /// The shared vtable for every Lua-backed component. Its ADDRESS is the
+    /// discriminator used by `EventBridge.releaseOverride` to recognize a
+    /// `Component` as bridged: a `Component` whose `vtable` pointer equals
+    /// `&BridgedComponent.vtable` is known to have `ptr == *BridgedComponent`.
+    /// Native (non-Lua) components carry a different vtable address, so the
+    /// release hook can safely leave them alone. `pub` so the discriminator is
+    /// addressable from the release path (and tests).
+    pub const vtable = Component.VTable{
+        .render = renderImpl,
+        .firstLineChanged = firstLineChangedImpl,
+        .invalidate = invalidateImpl,
+        .handleInput = handleInputImpl,
+    };
+
+    pub fn comp(self: *BridgedComponent) Component {
+        return .{ .ptr = self, .vtable = &vtable };
+    }
+};
+
+/// A registered Lua `on` handler, bridged to a native `Handler`.
+///
+/// `fn_ref` is a `luaL_ref` to the Lua handler function. The native
+/// `Handler.ctx` points at this struct; `Handler.callback` is
+/// `nativeHandlerCallback`, which builds the bridged `event` object, calls
+/// the Lua function, and reads back any component it set.
+pub const LuaHandler = struct {
+    bridge: *EventBridge,
+    fn_ref: c_int,
+    /// Bridge-owned copy of the event name this handler subscribed to.
+    event_name: []const u8,
+};
+
+/// The bridge: owns the Lua-side event wiring and the bridged
+/// components/handlers. Lives inside the `LuaRuntime`.
+pub const EventBridge = struct {
+    alloc: std.mem.Allocator,
+    L: *c.lua_State,
+    /// The native bus a Lua `emit` drives, and that fires Lua handlers.
+    /// Set once the App is constructed (`attachBus`); null before then, in
+    /// which case a Lua `emit` is a no-op (nothing to fire into yet).
+    bus: ?*EventBus = null,
+
+    /// Owned Lua handler bridges (one per `panto.ext.on` registration).
+    handlers: std.ArrayListUnmanaged(*LuaHandler) = .empty,
+    /// Owned bridged components, tracked so their refs + caches are freed
+    /// at teardown. Each is heap-allocated; an `invalidate` frees it.
+    components: std.ArrayListUnmanaged(*BridgedComponent) = .empty,
+
+    /// During a native `emit`, the event currently being presented to a
+    /// Lua handler. The bridged `event` userdata's methods read/write
+    /// through this. Single-threaded; valid only for the duration of one
+    /// handler call. `null` outside a handler.
+    active_event: ?*Event = null,
+
+    pub fn init(alloc: std.mem.Allocator, L: *c.lua_State) EventBridge {
+        return .{ .alloc = alloc, .L = L };
+    }
+
+    pub fn deinit(self: *EventBridge) void {
+        for (self.handlers.items) |h| {
+            c.luaL_unref(self.L, lua_bridge.LUA_REGISTRYINDEX, h.fn_ref);
+            self.alloc.free(h.event_name);
+            self.alloc.destroy(h);
+        }
+        self.handlers.deinit(self.alloc);
+        for (self.components.items) |comp| self.freeComponentInner(comp);
+        self.components.deinit(self.alloc);
+    }
+
+    /// Bind the native `EventBus` this bridge fires into / drives via
+    /// `emit`. Called once during startup wiring, after the App's bus
+    /// exists. Also registers every harvested Lua `on` handler into the
+    /// bus, in registration order.
+    pub fn attachBus(self: *EventBridge, bus: *EventBus) !void {
+        self.bus = bus;
+        for (self.handlers.items) |h| {
+            try bus.on(eventNameOf(h), .{ .ctx = h, .callback = nativeHandlerCallback });
+        }
+    }
+
+    /// The event name a handler subscribed to is stored alongside its ref
+    /// at registration time. We keep it on the LuaHandler via a parallel
+    /// slice; but to avoid a second allocation we look it up lazily. (Set
+    /// in `registerOnHandler`.)
+    fn eventNameOf(h: *LuaHandler) []const u8 {
+        return h.event_name;
+    }
+
+    /// Record a Lua `on` handler: `luaL_ref` the function at stack index
+    /// `fn_idx` and remember the event name. Registration into the bus
+    /// happens later in `attachBus` (the bus may not exist yet at harvest
+    /// time). Order of calls here is the registration order (§7.3).
+    pub fn registerOnHandler(self: *EventBridge, event_name: []const u8, fn_idx: c_int) !void {
+        // Dupe the event name into bridge-owned storage.
+        const name_copy = try self.alloc.dupe(u8, event_name);
+        errdefer self.alloc.free(name_copy);
+
+        // luaL_ref pops the top of stack, so push a copy of the function.
+        c.lua_pushvalue(self.L, fn_idx);
+        const ref = c.luaL_ref(self.L, lua_bridge.LUA_REGISTRYINDEX);
+        errdefer c.luaL_unref(self.L, lua_bridge.LUA_REGISTRYINDEX, ref);
+
+        const h = try self.alloc.create(LuaHandler);
+        h.* = .{ .bridge = self, .fn_ref = ref, .event_name = name_copy };
+        try self.handlers.append(self.alloc, h);
+    }
+
+    /// Number of registered Lua `on` handlers (diagnostic/test helper).
+    pub fn handlerCount(self: *const EventBridge) usize {
+        return self.handlers.items.len;
+    }
+
+    // -- bridged component construction ------------------------------------
+
+    /// Build a native `Component` backed by the Lua component table at
+    /// stack index `table_idx`. `luaL_ref`s the table, allocates a tracked
+    /// `BridgedComponent`, and returns its `comp()`. The component lives
+    /// until `invalidate` (which frees it) or runtime teardown.
+    ///
+    /// No "active component": each call mints a fresh instance.
+    fn makeBridgedComponent(self: *EventBridge, table_idx: c_int) !Component {
+        c.lua_pushvalue(self.L, table_idx);
+        const ref = c.luaL_ref(self.L, lua_bridge.LUA_REGISTRYINDEX);
+        errdefer c.luaL_unref(self.L, lua_bridge.LUA_REGISTRYINDEX, ref);
+
+        const bc = try self.alloc.create(BridgedComponent);
+        errdefer self.alloc.destroy(bc);
+        bc.* = .{ .bridge = self, .table_ref = ref, .cache = RenderCache.init(self.alloc) };
+        try self.components.append(self.alloc, bc);
+        return bc.comp();
+    }
+
+    /// Free a bridged component's Lua ref + cache and destroy it. Idempotent
+    /// via the `dead` flag, BUT note it does NOT remove `bc` from
+    /// `self.components` and it `destroy`s the allocation — so it must only be
+    /// used in one of two safe ways:
+    ///   - at teardown (`deinit`), iterating the whole list once; or
+    ///   - via `releaseOverride`, which removes `bc` from `self.components`
+    ///     BEFORE calling this, so teardown never revisits a destroyed pointer.
+    fn freeComponentInner(self: *EventBridge, bc: *BridgedComponent) void {
+        if (bc.dead) return;
+        bc.dead = true;
+        c.luaL_unref(self.L, lua_bridge.LUA_REGISTRYINDEX, bc.table_ref);
+        bc.cache.deinit();
+        self.alloc.destroy(bc);
+    }
+
+    /// Release a superseded override component handed back by the App's
+    /// mid-stream swap (`App.override_release_fn`). This is the leak-prevention
+    /// point: when a `tool_details` (or any lifecycle) handler swaps a NEW
+    /// component over a PRIOR Lua-backed override, the prior one must drop its
+    /// `luaL_ref` + `RenderCache`, or it leaks for the life of the runtime
+    /// (one leak per swapped tool call).
+    ///
+    /// Discriminator: a `Component` is Lua-backed iff its `vtable` pointer is
+    /// `&BridgedComponent.vtable` (the single shared vtable address; see
+    /// `BridgedComponent.vtable`). For such a component `ptr` is a
+    /// `*BridgedComponent` we own and track in `self.components`. Any other
+    /// vtable belongs to a NATIVE component the App owns; we must NOT touch it
+    /// (it is not ours to free), so the hook is a no-op for it.
+    ///
+    /// Ownership: we remove `bc` from `self.components` first, then free it, so
+    /// teardown's `deinit` loop never revisits the destroyed pointer.
+    pub fn releaseOverride(self: *EventBridge, old: Component) void {
+        if (old.vtable != &BridgedComponent.vtable) return; // native: not ours
+        const bc: *BridgedComponent = @ptrCast(@alignCast(old.ptr));
+        // Remove from the tracked list before freeing (swap-remove is fine;
+        // order does not matter for teardown).
+        for (self.components.items, 0..) |item, i| {
+            if (item == bc) {
+                _ = self.components.swapRemove(i);
+                break;
+            }
+        }
+        self.freeComponentInner(bc);
+    }
+
+    /// The `*anyopaque`-typed release callback the App invokes via
+    /// `override_release_fn`. `ctx` is the `*EventBridge`.
+    pub fn releaseOverrideThunk(ctx: *anyopaque, old: Component) void {
+        const self: *EventBridge = @ptrCast(@alignCast(ctx));
+        self.releaseOverride(old);
+    }
+
+    // -- bridged render / input (SYNCHRONOUS pcall) ------------------------
+
+    /// Render a Lua-defined component synchronously: push its table +
+    /// `render` method + width, `lua_pcallk`, marshal the returned
+    /// array-of-strings into the component's `RenderCache`, and return the
+    /// cached lines. On ANY Lua error or bad return, return a single dim
+    /// error line instead of crashing the render loop (the frame must
+    /// always complete). Every returned line is truncated to `width` so
+    /// the engine's width contract (§3.1) is never violated.
+    fn renderBridged(self: *EventBridge, bc: *BridgedComponent, width: usize) anyerror![]const []const u8 {
+        const L = self.L;
+        if (bc.dead) return &.{};
+
+        const base = c.lua_gettop(L);
+        defer c.lua_settop(L, base); // restore stack no matter what
+
+        // Push the component table, then its `render` method.
+        _ = c.lua_rawgeti(L, lua_bridge.LUA_REGISTRYINDEX, @intCast(bc.table_ref));
+        if (c.lua_type(L, -1) != lua_bridge.T_TABLE) return self.fallbackLines(bc, width, "component is not a table");
+        _ = c.lua_getfield(L, -1, "render");
+        if (c.lua_type(L, -1) != lua_bridge.T_FUNCTION) {
+            return self.fallbackLines(bc, width, "component has no render()");
+        }
+        // Stack: ..., table, render. Call render(table, width) -> lines.
+        c.lua_pushvalue(L, -2); // self (the table)
+        c.lua_pushinteger(L, @intCast(width));
+        if (c.lua_pcallk(L, 2, 1, 0, 0, null) != 0) {
+            return self.fallbackLines(bc, width, luaErrText(L));
+        }
+        // Stack top: the returned lines table.
+        const raw = lua_bridge.readLinesArray(L, -1, self.alloc) catch {
+            return self.fallbackLines(bc, width, "render() must return an array of strings");
+        };
+        defer {
+            for (raw) |ln| self.alloc.free(ln);
+            self.alloc.free(raw);
+        }
+        // Enforce the width contract: truncate every line to `width`
+        // columns. Build a transient view of truncated borrows for the
+        // cache to dupe.
+        var view: std.ArrayListUnmanaged([]const u8) = .empty;
+        defer view.deinit(self.alloc);
+        for (raw) |ln| {
+            const vis = components.truncateToCols(ln, width);
+            try view.append(self.alloc, vis);
+        }
+        try bc.cache.store(view.items);
+        return cacheLines(&bc.cache);
+    }
+
+    /// Build the cache's single dim fallback line for a render failure and
+    /// return it. Keeps the render loop alive on a misbehaving extension.
+    ///
+    /// The composed line MUST satisfy the engine's width contract (§3.1):
+    /// visible width <= `width`, or the engine rejects it with
+    /// `Error.LineOverflow` and crashes the very frame the fallback exists to
+    /// save. The dim escapes are zero visible width, so we build the PLAIN
+    /// error text, truncate THAT to `width` columns (codepoint-safe), and only
+    /// then wrap it in the dim style.
+    fn fallbackLines(self: *EventBridge, bc: *BridgedComponent, width: usize, why: []const u8) anyerror![]const []const u8 {
+        _ = self;
+        const dim = theme.default.fg(.dim);
+        // Plain (escape-free) error text first, into a scratch buffer.
+        var plain_buf: [256]u8 = undefined;
+        const plain = std.fmt.bufPrint(&plain_buf, "[lua component error: {s}]", .{why}) catch "[lua component error]";
+        // Truncate the visible text to the width contract (codepoint-safe).
+        const vis = components.truncateToCols(plain, width);
+        // Compose the styled line; if even the styled form overflows the
+        // compose buffer, fall back to the bare truncated plain text (which
+        // already satisfies the width contract).
+        var styled_buf: [512]u8 = undefined;
+        const styled = std.fmt.bufPrint(&styled_buf, "{s}{s}{s}", .{ dim.open(), vis, dim.close() }) catch vis;
+        const one = [_][]const u8{styled};
+        try bc.cache.store(&one);
+        return cacheLines(&bc.cache);
+    }
+
+    /// Feed input to a Lua component's optional `handleInput(self, data)`
+    /// method, synchronously. Errors are swallowed (input handling must
+    /// never crash the loop); a component without the method ignores input.
+    fn handleInputBridged(self: *EventBridge, bc: *BridgedComponent, data: []const u8) void {
+        const L = self.L;
+        if (bc.dead) return;
+        const base = c.lua_gettop(L);
+        defer c.lua_settop(L, base);
+
+        _ = c.lua_rawgeti(L, lua_bridge.LUA_REGISTRYINDEX, @intCast(bc.table_ref));
+        if (c.lua_type(L, -1) != lua_bridge.T_TABLE) return;
+        _ = c.lua_getfield(L, -1, "handleInput");
+        if (c.lua_type(L, -1) != lua_bridge.T_FUNCTION) return;
+        c.lua_pushvalue(L, -2); // self
+        _ = c.lua_pushlstring(L, data.ptr, data.len);
+        // Any error: ignore (best-effort input).
+        _ = c.lua_pcallk(L, 2, 0, 0, 0, null);
+
+        // Mark the cache dirty after input. The engine reads
+        // `firstLineChanged()` BEFORE calling `render`, so a clean cache means
+        // the component is SKIPPED and the Lua-side mutation would never reach
+        // the screen. Native components dirty on every input mutation (see
+        // InputBox.applyKey -> markDirty); the bridge must do the same on the
+        // component's behalf. We can't know what changed inside Lua, so we
+        // re-dirty from 0; the post-render diff in `store` then recovers the
+        // true lowest-changed line (preserving the streaming-tail property for
+        // append-style edits). Dirty unconditionally even on a Lua error: the
+        // handler may have partially mutated state before throwing.
+        bc.cache.markDirty();
+    }
+};
+
+// ===========================================================================
+// Native handler callback: native EventBus -> Lua `on` handler
+// ===========================================================================
+
+/// The native `Handler.callback` for a bridged Lua handler. Builds the
+/// `event` userdata, calls the Lua function with it (synchronously, under
+/// a traceback errfunc), and lets the Lua side read/replace the component
+/// via `event:getComponent()` / `event:setComponent()`. Errors in the Lua
+/// handler are logged and swallowed — a broken handler must not abort the
+/// event dispatch or the render loop.
+fn nativeHandlerCallback(ctx: *anyopaque, event: *Event) void {
+    const h: *LuaHandler = @ptrCast(@alignCast(ctx));
+    const self = h.bridge;
+    const L = self.L;
+
+    // Make this event the active one so the userdata methods read/write it.
+    const prev = self.active_event;
+    self.active_event = event;
+    defer self.active_event = prev;
+
+    const base = c.lua_gettop(L);
+    defer c.lua_settop(L, base);
+
+    // Traceback errfunc beneath the call.
+    _ = c.lua_getglobal(L, "debug");
+    _ = c.lua_getfield(L, -1, "traceback");
+    c.lua_copy(L, -1, -2);
+    c.lua_settop(L, c.lua_gettop(L) - 1); // pop `debug` table
+    const errfunc_idx = c.lua_gettop(L);
+
+    _ = c.lua_rawgeti(L, lua_bridge.LUA_REGISTRYINDEX, @intCast(h.fn_ref));
+    pushEventObject(self) catch {
+        c.lua_settop(L, base);
+        return;
+    };
+    if (c.lua_pcallk(L, 1, 0, errfunc_idx, 0, null) != 0) {
+        var len: usize = 0;
+        const msg = c.lua_tolstring(L, -1, &len);
+        if (msg != null) {
+            std.log.err("lua event handler '{s}' failed: {s}", .{ h.event_name, msg[0..len] });
+        }
+    }
+}
+
+// ===========================================================================
+// The bridged `event` object (userdata + metatable)
+// ===========================================================================
+
+/// Push an `event` userdata onto the stack, carrying a pointer to the
+/// bridge (which reaches the active `*Event`). Its metatable exposes
+/// `getComponent`, `setComponent`, and read-only payload fields via
+/// `__index`.
+fn pushEventObject(bridge: *EventBridge) !void {
+    const L = bridge.L;
+    const ud: **EventBridge = @ptrCast(@alignCast(c.lua_newuserdatauv(L, @sizeOf(*EventBridge), 0).?));
+    ud.* = bridge;
+    ensureEventMetatable(L);
+    _ = c.lua_setmetatable(L, -2);
+}
+
+/// Lazily create the `event` userdata metatable (by name, so
+/// `luaL_checkudata` recognizes it) and leave it on the stack. Its
+/// `__index` is a function resolving `getComponent`/`setComponent` and
+/// payload fields.
+fn ensureEventMetatable(L: *c.lua_State) void {
+    // luaL_newmetatable pushes the metatable; returns 1 if freshly created.
+    if (c.luaL_newmetatable(L, eventMtName) != 0) {
+        c.lua_pushcclosure(L, eventIndexThunk, 0);
+        c.lua_setfield(L, -2, "__index");
+    }
+}
+
+/// `__index(event_ud, key)`: dispatch method names and payload fields.
+fn eventIndexThunk(L_opt: ?*c.lua_State) callconv(.c) c_int {
+    const L = L_opt.?;
+    const ud: **EventBridge = @ptrCast(@alignCast(c.luaL_checkudata(L, 1, eventMtName) orelse return 0));
+    const bridge = ud.*;
+    var klen: usize = 0;
+    const kptr = c.lua_tolstring(L, 2, &klen);
+    if (kptr == null) return 0;
+    const key = kptr[0..klen];
+
+    if (std.mem.eql(u8, key, "getComponent")) {
+        c.lua_pushcclosure(L, eventGetComponentThunk, 0);
+        return 1;
+    }
+    if (std.mem.eql(u8, key, "setComponent")) {
+        c.lua_pushcclosure(L, eventSetComponentThunk, 0);
+        return 1;
+    }
+    // Payload fields + the event name.
+    const ev = bridge.active_event orelse {
+        c.lua_pushnil(L);
+        return 1;
+    };
+    if (std.mem.eql(u8, key, "name")) {
+        _ = c.lua_pushlstring(L, ev.name.ptr, ev.name.len);
+        return 1;
+    }
+    pushPayloadField(L, ev, key);
+    return 1;
+}
+
+/// Push the payload field named `key` for `ev`, or nil if absent. Marshals
+/// the tagged-union variant's fields onto simple Lua values (§7.2).
+/// Push a string field as a Lua string, or `nil` when the slice is empty.
+/// Empty lifecycle fields (e.g. `delta` at a non-delta boundary, `tool_name`
+/// before `tool_details`) read as nil on the Lua side, so handlers can branch
+/// on presence (`if event.tool_name then ...`).
+fn pushStringOrNil(L: *c.lua_State, s: []const u8) void {
+    if (s.len == 0) {
+        c.lua_pushnil(L);
+    } else {
+        _ = c.lua_pushlstring(L, s.ptr, s.len);
+    }
+}
+
+fn pushPayloadField(L: *c.lua_State, ev: *Event, key: []const u8) void {
+    switch (ev.payload) {
+        .tool => |t| {
+            // index/tool_name plus the lifecycle fields: `id` (resolved
+            // call id), `delta` (this args chunk on tool_delta), `input`
+            // (accumulated/final args), `output` (the result text on
+            // tool_result). String fields return nil when empty/absent so a
+            // handler can test `if event.delta then ...`.
+            if (std.mem.eql(u8, key, "index")) {
+                c.lua_pushinteger(L, @intCast(t.index));
+                return;
+            }
+            if (std.mem.eql(u8, key, "tool_name")) return pushStringOrNil(L, t.tool_name);
+            if (std.mem.eql(u8, key, "id")) return pushStringOrNil(L, t.id);
+            if (std.mem.eql(u8, key, "delta")) return pushStringOrNil(L, t.delta);
+            if (std.mem.eql(u8, key, "input")) return pushStringOrNil(L, t.input);
+            if (std.mem.eql(u8, key, "output")) return pushStringOrNil(L, t.output);
+        },
+        .thinking => |t| {
+            // index plus `delta` (this chunk on *_delta) and `text` (the
+            // accumulated/final buffer). Empty -> nil.
+            if (std.mem.eql(u8, key, "index")) {
+                c.lua_pushinteger(L, @intCast(t.index));
+                return;
+            }
+            if (std.mem.eql(u8, key, "delta")) return pushStringOrNil(L, t.delta);
+            if (std.mem.eql(u8, key, "text")) return pushStringOrNil(L, t.text);
+        },
+        .assistant_text => |t| {
+            // Same lifecycle fields as thinking.
+            if (std.mem.eql(u8, key, "index")) {
+                c.lua_pushinteger(L, @intCast(t.index));
+                return;
+            }
+            if (std.mem.eql(u8, key, "delta")) return pushStringOrNil(L, t.delta);
+            if (std.mem.eql(u8, key, "text")) return pushStringOrNil(L, t.text);
+        },
+        .user_message => |m| {
+            if (std.mem.eql(u8, key, "text")) {
+                _ = c.lua_pushlstring(L, m.text.ptr, m.text.len);
+                return;
+            }
+        },
+        .session_start => |s| {
+            if (std.mem.eql(u8, key, "version")) {
+                _ = c.lua_pushlstring(L, s.version.ptr, s.version.len);
+                return;
+            }
+            if (std.mem.eql(u8, key, "cwd")) {
+                _ = c.lua_pushlstring(L, s.cwd.ptr, s.cwd.len);
+                return;
+            }
+            if (std.mem.eql(u8, key, "model")) {
+                _ = c.lua_pushlstring(L, s.model.ptr, s.model.len);
+                return;
+            }
+        },
+        .compaction => |cm| {
+            if (std.mem.eql(u8, key, "summary")) {
+                _ = c.lua_pushlstring(L, cm.summary.ptr, cm.summary.len);
+                return;
+            }
+        },
+        .custom => {},
+    }
+    c.lua_pushnil(L);
+}
+
+/// `event:getComponent()` -> the current component as a native-passthrough
+/// userdata (or nil). The userdata wraps the `Component` (vtable+ptr) so
+/// Lua can pass it straight back to `setComponent` unchanged (§7.5 wrap
+/// pattern: the inner is opaque to Lua but re-settable / wrappable).
+fn eventGetComponentThunk(L_opt: ?*c.lua_State) callconv(.c) c_int {
+    const L = L_opt.?;
+    // arg 1 is the event userdata (method call `event:getComponent()`).
+    const ud: **EventBridge = @ptrCast(@alignCast(c.luaL_checkudata(L, 1, eventMtName) orelse return 0));
+    const bridge = ud.*;
+    const ev = bridge.active_event orelse {
+        c.lua_pushnil(L);
+        return 1;
+    };
+    if (ev.getComponent()) |comp| {
+        pushNativeComponent(L, comp);
+    } else {
+        c.lua_pushnil(L);
+    }
+    return 1;
+}
+
+/// `event:setComponent(c)` where `c` is EITHER a native-passthrough
+/// userdata (from `getComponent`) OR a Lua component table. A table is
+/// bridged into a native `Component` (§7.6); a userdata passes the native
+/// component straight through.
+fn eventSetComponentThunk(L_opt: ?*c.lua_State) callconv(.c) c_int {
+    const L = L_opt.?;
+    const ud: **EventBridge = @ptrCast(@alignCast(c.luaL_checkudata(L, 1, eventMtName) orelse return 0));
+    const bridge = ud.*;
+    const ev = bridge.active_event orelse return 0;
+
+    const ty = c.lua_type(L, 2);
+    if (ty == lua_bridge.T_USERDATA) {
+        // Native passthrough: recover the Component from the userdata.
+        if (c.luaL_testudata(L, 2, nativeCompMtName)) |raw| {
+            const cud: *Component = @ptrCast(@alignCast(raw));
+            ev.setComponent(cud.*);
+            return 0;
+        }
+        return c.luaL_error(L, "setComponent: unknown userdata (expected a component)");
+    }
+    if (ty == lua_bridge.T_TABLE) {
+        const comp = bridge.makeBridgedComponent(2) catch {
+            return c.luaL_error(L, "setComponent: failed to bridge Lua component");
+        };
+        ev.setComponent(comp);
+        return 0;
+    }
+    return c.luaL_error(L, "setComponent: argument must be a component (table or native handle)");
+}
+
+/// Push a native `Component` as a passthrough userdata with the
+/// native-component metatable (so `setComponent` can recover it).
+fn pushNativeComponent(L: *c.lua_State, comp: Component) void {
+    const ud: *Component = @ptrCast(@alignCast(c.lua_newuserdatauv(L, @sizeOf(Component), 0).?));
+    ud.* = comp;
+    ensureNativeCompMetatable(L);
+    _ = c.lua_setmetatable(L, -2);
+}
+
+fn ensureNativeCompMetatable(L: *c.lua_State) void {
+    // A bare named metatable (no methods); identity-only so
+    // `luaL_testudata` can recognize the passthrough handle.
+    _ = c.luaL_newmetatable(L, nativeCompMtName);
+}
+
+/// Read the top-of-stack Lua error as text (after a failed pcall).
+fn luaErrText(L: *c.lua_State) []const u8 {
+    var len: usize = 0;
+    const ptr = c.lua_tolstring(L, -1, &len);
+    if (ptr == null) return "lua error";
+    return ptr[0..len];
+}
+
+/// Re-type the cache's owned lines as `[]const []const u8` for the vtable
+/// return (mirrors the native components' `cacheLines`).
+fn cacheLines(cache: *RenderCache) []const []const u8 {
+    const owned = cache.lines orelse return &.{};
+    return @ptrCast(owned);
+}
+
+// ===========================================================================
+// emit: Lua `panto.ext.emit(name, data)` -> native EventBus
+// ===========================================================================
+
+/// C thunk installed as `panto.ext.emit` by the runtime (`installEmit`),
+/// carrying the `*EventBridge` as a light-userdata upvalue. Fires a custom
+/// event on the native bus so native AND Lua handlers run. The `data`
+/// argument is currently surfaced to handlers only as a `.custom` payload
+/// (opaque); structured custom-data marshalling can be added later. The
+/// chosen component (if any handler set one) is NOT auto-mounted here —
+/// imperative mounting from a bare `emit` is a future refinement; for now
+/// `emit` drives handler side effects and component selection for events
+/// the app fires at real component-creation boundaries.
+pub fn emitThunk(L_opt: ?*c.lua_State) callconv(.c) c_int {
+    const L = L_opt.?;
+    const self_ptr = c.lua_touserdata(L, c.lua_upvalueindex(1));
+    if (self_ptr == null) return 0;
+    const bridge: *EventBridge = @ptrCast(@alignCast(self_ptr.?));
+    const bus = bridge.bus orelse return 0; // no bus yet: no-op
+
+    var nlen: usize = 0;
+    const nptr = c.lua_tolstring(L, 1, &nlen);
+    if (nptr == null) return c.luaL_error(L, "emit: first argument must be an event name string");
+    const name = nptr[0..nlen];
+
+    var ev = Event.init(name, null, .{ .custom = .{} });
+    _ = bus.emit(&ev);
+    return 0;
+}
+
+// ===========================================================================
+// Tests
+// ===========================================================================
+
+const testing = std.testing;
+
+/// Test helper: harvest every `panto.ext.on` registration currently in the
+/// state's on-registrations table into `bridge`, in order. Mirrors what
+/// `LuaRuntime.harvestAndStoreOnHandlers` does, without the full runtime.
+fn harvestOnInto(bridge: *EventBridge) !void {
+    const L = bridge.L;
+    _ = c.lua_rawgetp(L, lua_bridge.LUA_REGISTRYINDEX, &lua_bridge.on_registrations_key);
+    defer c.lua_settop(L, c.lua_gettop(L) - 1);
+    const n: usize = @intCast(c.lua_rawlen(L, -1));
+    var i: usize = 1;
+    while (i <= n) : (i += 1) {
+        _ = c.lua_rawgeti(L, -1, @intCast(i));
+        defer c.lua_settop(L, c.lua_gettop(L) - 1);
+        _ = c.lua_getfield(L, -1, "event");
+        var elen: usize = 0;
+        const eptr = c.lua_tolstring(L, -1, &elen);
+        const name = if (eptr != null) eptr[0..elen] else "";
+        c.lua_settop(L, c.lua_gettop(L) - 1);
+        _ = c.lua_getfield(L, -1, "handler");
+        try bridge.registerOnHandler(name, -1);
+        c.lua_settop(L, c.lua_gettop(L) - 1);
+    }
+}
+
+fn runScript(L: *c.lua_State, src: [:0]const u8) !void {
+    if (c.luaL_loadstring(L, src.ptr) != 0 or c.lua_pcallk(L, 0, 0, 0, 0, null) != 0) {
+        var len: usize = 0;
+        const msg = c.lua_tolstring(L, -1, &len);
+        std.debug.print("lua error: {s}\n", .{msg[0..len]});
+        return error.LuaScriptFailed;
+    }
+}
+
+test "Lua on-handler fires through the native bus into Lua" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // A handler that records the tool_name it saw into a global.
+    try runScript(L,
+        \\seen = nil
+        \\panto.ext.on("tool", function(e) seen = e.tool_name end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+    try testing.expectEqual(@as(usize, 1), bridge.handlerCount());
+
+    // Fire a native tool event; the Lua handler should observe tool_name.
+    _ = bus.fire("tool", null, .{ .tool = .{ .index = 3, .tool_name = "skill" } });
+
+    _ = c.lua_getglobal(L, "seen");
+    var len: usize = 0;
+    const ptr = c.lua_tolstring(L, -1, &len);
+    try testing.expect(ptr != null);
+    try testing.expectEqualStrings("skill", ptr[0..len]);
+    c.lua_settop(L, c.lua_gettop(L) - 1);
+}
+
+test "Lua-defined component renders lines through the bridged vtable" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // A handler that sets a Lua component whose render returns two lines.
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({
+        \\    render = function(self, width) return { "hello", "world" } end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{ .index = 0, .tool_name = "x" } });
+    try testing.expect(chosen != null);
+
+    // Render the bridged component at width 80.
+    const lines = try chosen.?.render(80, testing.allocator);
+    try testing.expectEqual(@as(usize, 2), lines.len);
+    try testing.expectEqualStrings("hello", lines[0]);
+    try testing.expectEqualStrings("world", lines[1]);
+    // firstLineChanged is cache-derived: first render dirties from 0.
+    try testing.expectEqual(@as(?usize, 0), chosen.?.firstLineChanged());
+    // A second identical render reports no change.
+    _ = try chosen.?.render(80, testing.allocator);
+    try testing.expectEqual(@as(?usize, null), chosen.?.firstLineChanged());
+}
+
+test "bridged component render truncates to the width contract" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({
+        \\    render = function(self, width) return { "abcdefghij" } end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+    const lines = try chosen.render(4, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), lines.len);
+    // Truncated to 4 columns.
+    try testing.expectEqualStrings("abcd", lines[0]);
+}
+
+test "bridged component render error yields a safe fallback line, not a crash" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({
+        \\    render = function(self, width) error("boom") end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+    const lines = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), lines.len);
+    try testing.expect(std.mem.indexOf(u8, lines[0], "lua component error") != null);
+}
+
+test "wrap pattern: Lua reads the native default, wraps it, and renders through it" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // The native default: a component rendering one line "DEFAULT".
+    const NativeDefault = struct {
+        cache: RenderCache,
+        fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
+            _ = width;
+            _ = alloc;
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            const one = [_][]const u8{"DEFAULT"};
+            try self.cache.store(&one);
+            const owned = self.cache.lines orelse return &.{};
+            return @ptrCast(owned);
+        }
+        fn fcImpl(ptr: *anyopaque) ?usize {
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            return self.cache.firstLineChanged();
+        }
+        fn invImpl(ptr: *anyopaque) void {
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            self.cache.invalidate();
+        }
+        const vt = Component.VTable{ .render = renderImpl, .firstLineChanged = fcImpl, .invalidate = invImpl };
+        fn comp(self: *@This()) Component {
+            return .{ .ptr = self, .vtable = &vt };
+        }
+    };
+    var nd = NativeDefault{ .cache = RenderCache.init(testing.allocator) };
+    defer nd.cache.deinit();
+
+    // Handler: read the native default via getComponent, store it, set a Lua
+    // component that renders "[" .. inner_first_line .. "]".
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  local inner = e:getComponent()        -- native passthrough handle
+        \\  e:setComponent({
+        \\    inner = inner,
+        \\    render = function(self, width)
+        \\      -- We cannot call the native inner's render from Lua (it has no
+        \\      -- Lua method); the wrap here decorates around it. Prove we held
+        \\      -- the handle by setting it back is exercised by passthrough test.
+        \\      return { "wrapped" }
+        \\    end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", nd.comp(), .{ .tool = .{} }).?;
+    // The chosen component is the Lua wrapper, not the native default.
+    try testing.expect(chosen.ptr != nd.comp().ptr);
+    const lines = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), lines.len);
+    try testing.expectEqualStrings("wrapped", lines[0]);
+}
+
+test "skills-style claim-by-name (§7.5) works end-to-end through the Lua bridge" {
+    // The canonical extension pattern: a Lua handler subscribes to `tool`,
+    // returns early UNLESS event.tool_name matches its tool, and otherwise
+    // setComponent's a Lua-defined component. We fire two tool events through
+    // the NATIVE bus and assert only the matching name is claimed (its Lua
+    // component renders), while a non-matching name keeps the native default.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // NOTE: the `event` object is only valid DURING the handler call; a
+    // component must capture any payload it needs into its own state at handler
+    // time, NOT close over `e` and read it at render time. Here we snapshot the
+    // name into a local that the render closure captures.
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  if e.tool_name ~= "skill" then return end   -- claim-by-name
+        \\  local name = e.tool_name                    -- snapshot at handler time
+        \\  e:setComponent({
+        \\    render = function(self, width) return { "SKILL:" .. name } end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    // A native default the handler may or may not replace.
+    const Native = struct {
+        cache: RenderCache,
+        fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
+            _ = width;
+            _ = alloc;
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            const one = [_][]const u8{"NATIVE-DEFAULT"};
+            try self.cache.store(&one);
+            const owned = self.cache.lines orelse return &.{};
+            return @ptrCast(owned);
+        }
+        fn fcImpl(ptr: *anyopaque) ?usize {
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            return self.cache.firstLineChanged();
+        }
+        fn invImpl(ptr: *anyopaque) void {
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            self.cache.invalidate();
+        }
+        const vt = Component.VTable{ .render = renderImpl, .firstLineChanged = fcImpl, .invalidate = invImpl };
+        fn comp(self: *@This()) Component {
+            return .{ .ptr = self, .vtable = &vt };
+        }
+    };
+
+    // Non-matching name ("read"): the handler returns early, the default stays.
+    {
+        var nd = Native{ .cache = RenderCache.init(testing.allocator) };
+        defer nd.cache.deinit();
+        const chosen = bus.fire("tool", nd.comp(), .{ .tool = .{ .index = 0, .tool_name = "read" } }).?;
+        try testing.expectEqual(@as(*anyopaque, nd.comp().ptr), chosen.ptr); // unchanged
+        const lines = try chosen.render(80, testing.allocator);
+        try testing.expectEqualStrings("NATIVE-DEFAULT", lines[0]);
+    }
+
+    // Matching name ("skill"): the handler claims it with a Lua component.
+    {
+        var nd = Native{ .cache = RenderCache.init(testing.allocator) };
+        defer nd.cache.deinit();
+        const chosen = bus.fire("tool", nd.comp(), .{ .tool = .{ .index = 1, .tool_name = "skill" } }).?;
+        try testing.expect(chosen.ptr != nd.comp().ptr); // replaced by the Lua component
+        const lines = try chosen.render(80, testing.allocator);
+        try testing.expectEqual(@as(usize, 1), lines.len);
+        try testing.expectEqualStrings("SKILL:skill", lines[0]);
+    }
+}
+
+test "native-passthrough get/set round-trip: setComponent(getComponent()) keeps the native default" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    const Native = struct {
+        line_storage: [1][]const u8 = undefined,
+        fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
+            _ = width;
+            _ = alloc;
+            const self: *@This() = @ptrCast(@alignCast(ptr));
+            self.line_storage[0] = "N";
+            return self.line_storage[0..];
+        }
+        fn fcImpl(ptr: *anyopaque) ?usize {
+            _ = ptr;
+            return 0;
+        }
+        fn invImpl(ptr: *anyopaque) void {
+            _ = ptr;
+        }
+        const vt = Component.VTable{ .render = renderImpl, .firstLineChanged = fcImpl, .invalidate = invImpl };
+        fn comp(self: *@This()) Component {
+            return .{ .ptr = self, .vtable = &vt };
+        }
+    };
+    var native = Native{};
+
+    // Handler passes the native default straight back through.
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent(e:getComponent())
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", native.comp(), .{ .tool = .{} }).?;
+    // Round-trip preserved the SAME native component pointer.
+    try testing.expectEqual(@as(*anyopaque, native.comp().ptr), chosen.ptr);
+}
+
+test "Lua emit drives the native bus (custom event reaches a native handler)" {
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // Install the real emit (carrying the bridge) so Lua emit reaches the bus.
+    lua_bridge.installEmit(L, @ptrCast(&bridge), emitThunk);
+    try bridge.attachBus(&bus);
+
+    // A NATIVE handler that flips a flag when "custom-thing" fires.
+    const Flag = struct {
+        hit: bool = false,
+        fn cb(ctx: *anyopaque, ev: *Event) void {
+            _ = ev;
+            const self: *@This() = @ptrCast(@alignCast(ctx));
+            self.hit = true;
+        }
+    };
+    var flag = Flag{};
+    try bus.on("custom-thing", .{ .ctx = &flag, .callback = Flag.cb });
+
+    // Fire it from Lua.
+    try runScript(L, "panto.ext.emit(\"custom-thing\", {})");
+    try testing.expect(flag.hit);
+}
+
+test "bridged render: a long error on a NARROW width still satisfies the width contract" {
+    // Regression: the safe fallback line must itself obey the engine's width
+    // contract (visible width <= width). Otherwise the engine rejects it with
+    // LineOverflow and crashes the exact frame the fallback exists to save.
+    // We render a crashing component at a width far narrower than the error
+    // text and assert the produced line's visible width fits.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({
+        \\    render = function(self, width) error("a very long error message that definitely exceeds a narrow terminal width") end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+    const width: usize = 10;
+    const lines = try chosen.render(width, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), lines.len);
+    // The crux: visible width must fit, even though the raw error is long.
+    try testing.expect(engine.visibleWidth(lines[0]) <= width);
+}
+
+test "bridged render: non-array / nil / non-string returns each yield a safe fallback" {
+    // render() must return an array of strings. A table-with-non-strings, a
+    // bare nil, and a non-table scalar each route to the fallback line rather
+    // than crashing or leaking.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // Three components, each returning a malformed render value.
+    try runScript(L,
+        \\components = {
+        \\  function(self, width) return { {} } end,   -- array of a table (non-string)
+        \\  function(self, width) return nil end,       -- nil
+        \\  function(self, width) return 42 end,        -- non-table scalar
+        \\}
+        \\idx = 0
+        \\panto.ext.on("tool", function(e)
+        \\  idx = idx + 1
+        \\  e:setComponent({ render = components[idx] })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    inline for (0..3) |_| {
+        const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+        const lines = try chosen.render(80, testing.allocator);
+        try testing.expectEqual(@as(usize, 1), lines.len);
+        try testing.expect(std.mem.indexOf(u8, lines[0], "lua component error") != null);
+    }
+}
+
+test "bridged render: empty array renders zero lines (no fallback)" {
+    // An empty table is a VALID render result (zero lines), distinct from a
+    // malformed return. It must produce no lines and no error.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, width) return {} end })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+    const lines = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(usize, 0), lines.len);
+}
+
+test "bridged render: UTF-8 line truncates on codepoint boundaries to the width" {
+    // The width contract counts visible columns as codepoints. A multibyte
+    // line must truncate on a codepoint boundary, never mid-sequence, and the
+    // result's visible width must fit.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // "ééééé" is 5 codepoints, 10 bytes. Truncating to 3 columns must yield
+    // exactly 3 codepoints (6 bytes), not split a 2-byte sequence.
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, width) return { "\195\169\195\169\195\169\195\169\195\169" } end })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+    const lines = try chosen.render(3, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), lines.len);
+    try testing.expectEqual(@as(usize, 3), engine.visibleWidth(lines[0]));
+    // 3 codepoints * 2 bytes each = 6 bytes, intact.
+    try testing.expectEqual(@as(usize, 6), lines[0].len);
+}
+
+test "bridged firstLineChanged is cache-derived: append stays near the tail" {
+    // The bridge owns a native RenderCache, so a streaming component that
+    // appends a line reports firstLineChanged near the TAIL (the append
+    // boundary), not 0 — the same streaming-tail property native components
+    // get. We drive a Lua component whose render output grows by one line.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // `n` lines on each render, controlled by a global the test bumps.
+    try runScript(L,
+        \\n = 2
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({
+        \\    render = function(self, width)
+        \\      local t = {}
+        \\      for i = 1, n do t[i] = "line" .. i end
+        \\      return t
+        \\    end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+
+    // First render: 2 lines, dirty from 0.
+    _ = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(?usize, 0), chosen.firstLineChanged());
+    // No change: null.
+    _ = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(?usize, null), chosen.firstLineChanged());
+
+    // Append a 3rd line: the cache diff reports the boundary (index 2), NOT 0.
+    try runScript(L, "n = 3");
+    chosen.invalidate(); // re-dirty so render recomputes
+    _ = try chosen.render(80, testing.allocator);
+    // invalidate() drops to a full re-dirty (from 0), so this proves the cache
+    // path is wired; the precise tail-index is covered by native RenderCache
+    // tests. Re-render once more with no change to confirm it settles to null.
+    _ = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(?usize, null), chosen.firstLineChanged());
+}
+
+test "bridged firstLineChanged: a mid-line replace reports the changed line, shrink reports the boundary" {
+    // Cache-derived diff on REPLACE and SHRINK, mirroring native RenderCache
+    // expectations through the bridge.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\lines = { "a", "b", "c" }
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, width) return lines end })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+
+    _ = try chosen.render(80, testing.allocator); // dirty from 0
+    _ = try chosen.render(80, testing.allocator); // null
+    try testing.expectEqual(@as(?usize, null), chosen.firstLineChanged());
+
+    // Replace the MIDDLE line. Diff must report index 1.
+    try runScript(L, "lines = { \"a\", \"B\", \"c\" }");
+    chosen.invalidate();
+    _ = try chosen.render(80, testing.allocator);
+    // After invalidate the first render dirties from 0; settle, then mutate
+    // WITHOUT invalidate to exercise the pure diff path is not possible here
+    // (the bridge re-dirties only via invalidate/markDirty). The diff index on
+    // a settled-then-changed render is covered natively; here we confirm a
+    // shrink settles cleanly.
+    _ = try chosen.render(80, testing.allocator);
+
+    // Shrink to 1 line.
+    try runScript(L, "lines = { \"a\" }");
+    chosen.invalidate();
+    _ = try chosen.render(80, testing.allocator);
+    const shrunk = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), shrunk.len);
+    try testing.expectEqualStrings("a", shrunk[0]);
+}
+
+test "bridged handleInput round-trips: a Lua method mutates state the next render reflects" {
+    // handleInput(self, data) is bridged synchronously. A component that
+    // accumulates input bytes must show them on the subsequent render.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({
+        \\    buf = "",
+        \\    handleInput = function(self, data) self.buf = self.buf .. data end,
+        \\    render = function(self, width) return { self.buf } end,
+        \\  })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+    const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+
+    // Render once and settle the cache to clean (firstLineChanged == null).
+    _ = try chosen.render(80, testing.allocator);
+    _ = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(?usize, null), chosen.firstLineChanged());
+
+    // After input, the cache MUST be dirty so the engine (which reads
+    // firstLineChanged BEFORE render) actually re-renders the component.
+    chosen.handleInput("he");
+    try testing.expect(chosen.firstLineChanged() != null);
+    chosen.handleInput("llo");
+    try testing.expect(chosen.firstLineChanged() != null);
+
+    const lines = try chosen.render(80, testing.allocator);
+    try testing.expectEqual(@as(usize, 1), lines.len);
+    try testing.expectEqualStrings("hello", lines[0]);
+}
+
+test "bridged component: invalidate frees the ref+cache eagerly; teardown is leak-free" {
+    // invalidate() on the COMPONENT vtable maps to cache.invalidate (re-dirty),
+    // NOT a free — freeing happens at teardown for all tracked components. This
+    // test sets several Lua components, renders them, and relies on the leak-
+    // checked test allocator: if any ref or cache leaks, the allocator reports
+    // it at deinit and the test fails.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit(); // frees all tracked BridgedComponents
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, width) return { "x", "y" } end })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    // Create several distinct bridged components (each fire makes a new one).
+    var i: usize = 0;
+    while (i < 5) : (i += 1) {
+        const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
+        _ = try chosen.render(40, testing.allocator);
+        chosen.invalidate(); // re-dirty; must not leak the cache on next render
+        _ = try chosen.render(40, testing.allocator);
+    }
+    // bridge.deinit() (deferred) frees every tracked component's ref + cache;
+    // the leak-checked allocator asserts no leaks.
+}
+
+test "lifecycle payload fields marshal to Lua: tool id/delta/input/output, thinking/assistant delta/text" {
+    // Every new lifecycle field must be readable from a Lua handler, and an
+    // empty field must read as nil so handlers can branch on presence.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // Handlers stash what they saw into Lua globals we then assert on.
+    try runScript(L,
+        \\seen = {}
+        \\panto.ext.on("tool_delta", function(e)
+        \\  seen.delta = e.delta; seen.tn = e.tool_name; seen.idx = e.index
+        \\end)
+        \\panto.ext.on("tool_call_complete", function(e)
+        \\  seen.input = e.input; seen.id = e.id
+        \\end)
+        \\panto.ext.on("tool_result", function(e)
+        \\  seen.output = e.output; seen.rid = e.id
+        \\end)
+        \\panto.ext.on("thinking_delta", function(e)
+        \\  seen.t_delta = e.delta; seen.t_text = e.text
+        \\end)
+        \\panto.ext.on("assistant_text_complete", function(e)
+        \\  seen.a_text = e.text; seen.a_delta = e.delta  -- delta empty -> nil
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    _ = bus.fire("tool_delta", null, .{ .tool = .{ .index = 7, .tool_name = "read", .delta = "{\"p" } });
+    _ = bus.fire("tool_call_complete", null, .{ .tool = .{ .index = 7, .tool_name = "read", .id = "call_1", .input = "{\"path\":\"x\"}" } });
+    _ = bus.fire("tool_result", null, .{ .tool = .{ .index = 7, .id = "call_1", .output = "file contents" } });
+    _ = bus.fire("thinking_delta", null, .{ .thinking = .{ .index = 2, .delta = "hm", .text = "hm" } });
+    _ = bus.fire("assistant_text_complete", null, .{ .assistant_text = .{ .index = 0, .text = "done" } });
+
+    // Assert via Lua, surfacing failures as a thrown Lua error.
+    try runScript(L,
+        \\assert(seen.delta == "{\"p", "tool_delta.delta")
+        \\assert(seen.tn == "read", "tool_delta.tool_name")
+        \\assert(seen.idx == 7, "tool_delta.index")
+        \\assert(seen.input == "{\"path\":\"x\"}", "tool_call_complete.input")
+        \\assert(seen.id == "call_1", "tool_call_complete.id")
+        \\assert(seen.output == "file contents", "tool_result.output")
+        \\assert(seen.rid == "call_1", "tool_result.id")
+        \\assert(seen.t_delta == "hm" and seen.t_text == "hm", "thinking_delta")
+        \\assert(seen.a_text == "done", "assistant_text_complete.text")
+        \\assert(seen.a_delta == nil, "empty delta must be nil")
+    );
+}
+
+test "lifecycle handlers fire for every new event name" {
+    // Confirm dispatch is purely by event-name string: a handler on each new
+    // lifecycle event name actually runs. No new dispatch machinery exists;
+    // this guards that the names are routed.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\fired = {}
+        \\for _, name in ipairs({
+        \\  "tool", "tool_details", "tool_delta", "tool_call_complete", "tool_result",
+        \\  "thinking", "thinking_delta", "thinking_complete",
+        \\  "assistant_text", "assistant_text_delta", "assistant_text_complete",
+        \\}) do
+        \\  panto.ext.on(name, function(e) fired[name] = (fired[name] or 0) + 1 end)
+        \\end
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    _ = bus.fire("tool", null, .{ .tool = .{} });
+    _ = bus.fire("tool_details", null, .{ .tool = .{ .tool_name = "x", .id = "c" } });
+    _ = bus.fire("tool_delta", null, .{ .tool = .{ .delta = "a" } });
+    _ = bus.fire("tool_call_complete", null, .{ .tool = .{ .input = "{}" } });
+    _ = bus.fire("tool_result", null, .{ .tool = .{ .output = "r" } });
+    _ = bus.fire("thinking", null, .{ .thinking = .{} });
+    _ = bus.fire("thinking_delta", null, .{ .thinking = .{ .delta = "t" } });
+    _ = bus.fire("thinking_complete", null, .{ .thinking = .{ .text = "t" } });
+    _ = bus.fire("assistant_text", null, .{ .assistant_text = .{} });
+    _ = bus.fire("assistant_text_delta", null, .{ .assistant_text = .{ .delta = "a" } });
+    _ = bus.fire("assistant_text_complete", null, .{ .assistant_text = .{ .text = "a" } });
+
+    try runScript(L,
+        \\for _, name in ipairs({
+        \\  "tool", "tool_details", "tool_delta", "tool_call_complete", "tool_result",
+        \\  "thinking", "thinking_delta", "thinking_complete",
+        \\  "assistant_text", "assistant_text_delta", "assistant_text_complete",
+        \\}) do
+        \\  assert(fired[name] == 1, "handler did not fire exactly once for " .. name)
+        \\end
+    );
+}
+
+test "claim-by-name at tool_details swaps over the start default; releasing the prior override is leak-free" {
+    // The revised lifecycle: `tool` fires at block_start (name unknown), a Lua
+    // handler may set a placeholder; then `tool_details` fires with the name
+    // and a handler claims the call by name and swaps a NEW Lua component over
+    // the prior one. The superseded override is handed to releaseOverride,
+    // which must drop its luaL_ref + cache (the leak-prevention path). The
+    // leak-checked test allocator asserts no leak across the swap.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    try runScript(L,
+        \\-- At block_start the name is unknown; set a placeholder Lua component.
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, w) return { "tool (?)" } end })
+        \\end)
+        \\-- At tool_details, claim by name and swap in the real component.
+        \\panto.ext.on("tool_details", function(e)
+        \\  if e.tool_name ~= "skill" then return end
+        \\  e:setComponent({ render = function(self, w) return { "SKILL" } end })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    // block_start: handler sets the placeholder Lua component.
+    const start = bus.fire("tool", null, .{ .tool = .{ .index = 0 } }).?;
+    {
+        const lines = try start.render(40, testing.allocator);
+        try testing.expectEqual(@as(usize, 1), lines.len);
+        try testing.expectEqualStrings("tool (?)", lines[0]);
+    }
+    // Two distinct Lua components were created so far would leak if not freed;
+    // track the count before the swap.
+    try testing.expectEqual(@as(usize, 1), bridge.components.items.len);
+
+    // tool_details: seed the event with the CURRENT override (the placeholder),
+    // mirroring how the App seeds getComponent with the current component. The
+    // handler claims "skill" and swaps in "SKILL".
+    const details = bus.fire("tool_details", start, .{ .tool = .{ .index = 0, .tool_name = "skill", .id = "c0" } }).?;
+    try testing.expect(details.ptr != start.ptr); // a NEW component
+    {
+        const lines = try details.render(40, testing.allocator);
+        try testing.expectEqualStrings("SKILL", lines[0]);
+    }
+    // Now two bridged components are tracked (placeholder + skill).
+    try testing.expectEqual(@as(usize, 2), bridge.components.items.len);
+
+    // The App's mid-stream swap hands the SUPERSEDED override (the placeholder
+    // `start`) back to the release hook. Exercise it directly.
+    bridge.releaseOverride(start);
+    // The placeholder is freed and untracked; only the skill component remains.
+    try testing.expectEqual(@as(usize, 1), bridge.components.items.len);
+
+    // The surviving component still renders correctly after the release.
+    const lines = try details.render(40, testing.allocator);
+    try testing.expectEqualStrings("SKILL", lines[0]);
+
+    // releaseOverride on a NATIVE component is a no-op (different vtable).
+    const Native = struct {
+        cache: RenderCache,
+        fn r(ptr: *anyopaque, w: usize, a: std.mem.Allocator) anyerror![]const []const u8 {
+            _ = w;
+            _ = a;
+            const s: *@This() = @ptrCast(@alignCast(ptr));
+            const one = [_][]const u8{"N"};
+            try s.cache.store(&one);
+            return @ptrCast(s.cache.lines orelse return &.{});
+        }
+        fn fc(ptr: *anyopaque) ?usize {
+            const s: *@This() = @ptrCast(@alignCast(ptr));
+            return s.cache.firstLineChanged();
+        }
+        fn inv(ptr: *anyopaque) void {
+            const s: *@This() = @ptrCast(@alignCast(ptr));
+            s.cache.invalidate();
+        }
+        const vt = Component.VTable{ .render = r, .firstLineChanged = fc, .invalidate = inv };
+        fn comp(s: *@This()) Component {
+            return .{ .ptr = s, .vtable = &vt };
+        }
+    };
+    var native = Native{ .cache = RenderCache.init(testing.allocator) };
+    defer native.cache.deinit();
+    bridge.releaseOverride(native.comp()); // must NOT touch our tracked list
+    try testing.expectEqual(@as(usize, 1), bridge.components.items.len);
+    // bridge.deinit() frees the remaining skill component; allocator asserts no leak.
+}
+
+test "intra-emit clobber: two handlers each mint a Lua component in ONE emit; both are tracked and freed at teardown (no true leak)" {
+    // §7.3 "last-wins-blind": when two handlers for the SAME event each call
+    // setComponent within a single emit, the bus keeps only the last as
+    // `current`. The App records only that last one as the entry's override,
+    // so the FIRST handler's freshly-minted Lua component is never handed to
+    // `releaseOverride` (the App never sees it). It is therefore NOT released
+    // mid-stream — it lingers in `bridge.components` until teardown.
+    //
+    // This test pins that behavior precisely:
+    //   - it is NOT a true leak: `bridge.deinit` frees every tracked component
+    //     (the leak-checked allocator below would fail otherwise);
+    //   - but it IS per-emit accumulation: a clobbering handler chain grows
+    //     `bridge.components` by one orphan per clobber for the runtime's life.
+    //
+    // The documented mitigation is the §7.3 wrap pattern (getComponent ->
+    // wrap -> setComponent), under which no component is orphaned because each
+    // handler decorates the current one instead of minting a rival. A handler
+    // that clobbers blind is at fault per the plan; the resource is still
+    // reclaimed at teardown, so correctness (no UAF / no true leak) holds.
+    const L = c.luaL_newstate() orelse return error.LuaInitFailed;
+    defer c.lua_close(L);
+    c.luaL_openlibs(L);
+    lua_bridge.install(L);
+
+    var bridge = EventBridge.init(testing.allocator, L);
+    defer bridge.deinit();
+    var bus = EventBus.init(testing.allocator);
+    defer bus.deinit();
+
+    // Two blind-clobber handlers for the same event: each mints its own Lua
+    // component, ignoring the current one.
+    try runScript(L,
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, w) return { "FIRST" } end })
+        \\end)
+        \\panto.ext.on("tool", function(e)
+        \\  e:setComponent({ render = function(self, w) return { "SECOND" } end })
+        \\end)
+    );
+    try harvestOnInto(&bridge);
+    try bridge.attachBus(&bus);
+
+    const chosen = bus.fire("tool", null, .{ .tool = .{ .index = 0 } }).?;
+    // The LAST handler won.
+    {
+        const lines = try chosen.render(40, testing.allocator);
+        try testing.expectEqualStrings("SECOND", lines[0]);
+    }
+    // BOTH components were minted and tracked: the first is now an orphan that
+    // the App can never release (it only saw `chosen`). It is reclaimed only
+    // at teardown.
+    try testing.expectEqual(@as(usize, 2), bridge.components.items.len);
+
+    // bridge.deinit() (deferred) frees BOTH — the leak-checked allocator proves
+    // there is no TRUE leak even though the orphan was never released mid-stream.
+}