diag(asyncify): write-time instrumentation + local warm-load repro findings

The prod differential ladder finished: staged byte VOLUME on a warm load is
the only trigger left (V1a siblings-without-lib-tables dies, V1b +120 files
survives, V1c sibling KiCad files renamed byte-for-byte dies, V1d Leonardo +
123MB of inert markdown dies on loads 3-4; 14MB never dies). 3D models,
collab/ydoc/presence, lib tables, sibling KiCad handling and file count are
all exonerated — volume only loads the dice on the underlying race.

That made the crash reproducible locally for the first time in six campaigns:
a persistent browser profile + a 110MB project fails every warm load with the
exact prod signature. Iteration is now ~12 minutes instead of a release cycle.

Shim: every fiber switch now records the departing side's remaining asyncify
buffer and its recorded rewind entry (rem=/rf=), which is what identified the
unrewindable capture and disproved buffer overflow. The deferral family is
closed for good — a microtask-deferred retry on a clean empty stack died
identically to the nested rewind, because the suspension is broken at write
time, not by nesting.

Shell: log the origin stack when wx reports the top window destroyed. That
notification fires from ~wxTopLevelWindowWasm for ANY top-level window, so a
transient frame dying mid-load navigates the user out of the editor — a real
bug in its own right, found while chasing the empty flight-recorder dumps.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019SE4o46Lnq3hF574FFq8x4

web/standalone/src/components/WasmTool.tsx

@@ -454,6 +454,11 @@ function markDeliberateNavigation() {
 const quitDispatcher = () => {
   if (quitHandled) return;
   quitHandled = true;
+  // The wasm side only calls this when the app's top window is genuinely
+  // destroyed. When that happens unexpectedly (2026-08-03: a guarded-off
+  // settle-window dispatch cascaded into a silent frame close), the stack is
+  // the only artifact that says WHO closed it — keep it in every log.
+  console.warn("[quit] wxAppTopWindowClosed invoked — top window destroyed", new Error("quit-origin").stack);
   activeQuitHook?.();
 };