fix: eliminate scrollback-clearing flash when streaming with expanded tool output

The root cause was a two-part bug:

1. Engine slot management (tui_engine.zig): The app rebuilds its full
   component list on every new transcript entry. The old approach drained
   every slot then re-added them all, resetting every component's render
   baseline. When earlier content had scrolled into native scrollback
   (viewport_top > 0), the engine's viewport-escape rule escalated any
   cut above the viewport top to a scrollback-clearing full redraw — the
   visible flash on every streamed block while a tool call was expanded.

   Fix: replace drain+rebuild with syncComponents(), which reconciles the
   live slot list in place by finding the longest common prefix and suffix
   (matched by component pointer). Prefix and suffix slots keep their
   baselines verbatim; only the changed region in the middle gets new
   empty slots. A pure insertion (the dominant case: a new transcript
   entry appended just before the pinned input+footer suffix) stays on
   the differential path with no clear. A structural change (slot
   replaced or removed) sets force_full for correctness.

2. RenderCache dirty signal (tui_component.zig): firstLineChanged()
   was returning the retained changed_from diff index even when the cache
   was clean. A first-paint store records changed_from == 0, so a clean
   but previously-rendered component was pegging the differential cut to
   line 0 on every subsequent frame, making unchanged slots above the
   insertion point drag the cut all the way up. Fix: when the cache is
   clean, firstLineChanged() returns null unconditionally. The recorded
   changed_from is now internal render bookkeeping only, not a live
   signal.

Additional changes:
- ToolUse.collapsed_tail_lines raised from 5 to 8 lines.
- ToolUse collapsed-view test updated to use 12 output lines and
  collapsed_tail_lines symbolically so the numbers self-document.
- tui_app.zig: override-slot test comment and assertion clarified to
  reflect that the override owns the slot pointer (not just renders it),
  and the SWAPPED-AT-DETAILS render check replaced with a pointer-equality
  check.
- lua_event_bridge.zig: minor related fixes.
- New tests: syncComponents middle-insertion no-clear, syncComponents
  structural removal forces full, syncComponents override swap; streaming
  expanded-tools no-clear regression test in tui_app.

1 files changed, 9 insertions, 6 deletions

diff --git a/src/lua_event_bridge.zig b/src/lua_event_bridge.zig
index f6d67e0..c4acb1d 100644
--- a/src/lua_event_bridge.zig
+++ b/src/lua_event_bridge.zig
@@ -797,9 +797,11 @@ test "Lua-defined component renders lines through the bridged vtable" {
     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.
+    // firstLineChanged is cache-derived and is the LIVE signal: after a render
+    // the cache is clean, so it reports null (the first-render diff index 0 is
+    // retained internally as bookkeeping only).
+    try testing.expectEqual(@as(?usize, null), chosen.?.firstLineChanged());
+    // A second identical render still reports no change.
     _ = try chosen.?.render(80, testing.allocator);
     try testing.expectEqual(@as(?usize, null), chosen.?.firstLineChanged());
 }
@@ -1258,10 +1260,11 @@ test "bridged firstLineChanged is cache-derived: append stays near the tail" {
     try bridge.attachBus(&bus);
     const chosen = bus.fire("tool", null, .{ .tool = .{} }).?;
 
-    // First render: 2 lines, dirty from 0.
+    // First render: 2 lines. After the render the cache is clean, so the live
+    // signal is null (the first-render diff index 0 is internal bookkeeping).
     _ = try chosen.render(80, testing.allocator);
-    try testing.expectEqual(@as(?usize, 0), chosen.firstLineChanged());
-    // No change: null.
+    try testing.expectEqual(@as(?usize, null), chosen.firstLineChanged());
+    // No change: still null.
     _ = try chosen.render(80, testing.allocator);
     try testing.expectEqual(@as(?usize, null), chosen.firstLineChanged());