Validation-pass fixes; DESIGN.md describes the shipped seam

jobs: a child whose wake pipe cannot be armed is refused up front instead of
started into a state nothing can wake (the luv-less drain loop survives only
for hosts without a loop); a closing child keeps its concurrency slot and
its id's exclusivity until the pump actually exits, so teardown can no
longer over-admit new children or let two turns share one session file.
Resume-metadata reads honor the plain-error contract on a malformed store.

workflow: the built-in schema subset validator is the only validator — the
jsonschema probe made behavior depend on an undeclared rock (see rockspec:
that dependency is deliberately rejected); dead exports and the unreachable
half of the structured-output guard are gone, keeping the empty-arguments
provider case.

DESIGN.md's seam sections now describe the shipped division: binding-level
async jobs and tool control, host-level resolve_model/ExtHost/turn events/
component handles, rock-level policy; protocol bodies run to completion on
the loop thread and cannot yield, cancellation is scoped to the stream that
opened it, and a child's compaction leaves protocol sessions alone.

1 files changed, 24 insertions, 0 deletions

diff --git a/spec/test_run.lua b/spec/test_run.lua
index 706faff..e4bb038 100644
--- a/spec/test_run.lua
+++ b/spec/test_run.lua
@@ -254,6 +254,30 @@ return {
         end)
     end },
 
+    -- The binding reports a stored record it cannot decode by raising. A
+    -- malformed message is skipped, and nothing here raises out to the tool.
+    { "stored metadata that cannot be decoded is skipped, not raised", function()
+        with_host(function(handle, profiles)
+            local stored = stored_reviewer()
+            for _, message in ipairs(stored) do
+                if message.metadata then
+                    message.metadata = nil
+                    message.metadata_error = "message_metadata: invalid JSON"
+                end
+            end
+            handle.add_session("0198-child", stored)
+            handle.queue({ output = "second turn" })
+
+            local text = run.handle({ id = "0198-child", prompt = "carry on" }, profiles)
+            assert(not text:find("Error:", 1, true), "a malformed record is not a tool error:\n" .. text)
+            has(text, "second turn")
+            has(text, "agent: ?") -- an unreadable manifest names no agent
+            assert(handle.spawns[1].model == "anthropic:sonnet",
+                "an unreadable stored default falls back to the primary's model, got " ..
+                tostring(handle.spawns[1].model))
+        end)
+    end },
+
     { "a resumed child takes its agent name from the manifest", function()
         with_host(function(handle, profiles)
             handle.add_session("0198-child", stored_reviewer())