fix(drift-trio): phase E — serialized fiber queue (#10a) + fiber-busy probes

runOnFiber now runs bodies strictly one-at-a-time through a park-safe FIFO
(collab_common.h): the per-body fire-and-forget coroutine interleaved under
load — an asyncify park inside commit.Push let the event loop start the next
body, so a local commit and a remote apply ran interleaved on shared state
(s_applyingRemote is one global), silently losing applies on the actively-
editing receiver (fuzz finding #10a; B now fuzzes clean; 39-test suite green).

kicadCollabFiberBusy embind probe (merged + standalone registrations): a
bare-embind-stack scratch save during a parked fiber mis-dispatches (table
index OOB) — trio.ts modelText/drift and production drift-detect now defer
while fiber work is in flight (#10b hardening; the trap's root cause is still
open and needs a symbolized stack — fuzz stays fixme'd).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G5cAM9M6q34n5X4dbrfVvi

web/standalone/src/wasm/collab/drift-detect.ts

@@ -125,6 +125,11 @@ export function startDriftDetection(opts: DriftDetectOptions): DriftDetector {
   // unchanged since the last report, or the session cap is spent). Being fully
   // synchronous is what lets the session-end check finish during page unload.
   function computeDrift(): DriftReportBody | null {
+    // Defer while collab fiber work is in flight (0008 finding #10b): a save
+    // on the bare embind stack during a parked apply fiber mis-dispatches
+    // (table index OOB). The next Y-update trigger retries.
+    const busy = (opts.mod as { kicadCollabFiberBusy?: () => boolean }).kicadCollabFiberBusy;
+    if (busy?.()) return null;
     save(scratchPath);
     let text: unknown;
     try {