feat(embedding): MT5 — deferred embedding with durable markers; write acks never wait on a neural net

A3 of the service-class pair (BRAINY-PROD-LATENCY-TRIAD): a VFS file write
ran the embedder synchronously while the caller waited — 5.6s p50 / 21.4s
p95 per small file on a production deployment, the dominant stage of every
capture write.

- add()/update() gain deferEmbedding: the write acks at durability (data +
  metadata persisted, a DURABLE pending marker under
  _system/pending_embeds/<id> written BEFORE the commit — orphan-safe
  direction); the single-flight background worker embeds the CURRENT data
  and swaps the vector in ATOMICALLY (ReplaceInVectorIndex — the row is
  never absent from search; a deferred UPDATE keeps serving the OLD vector,
  stale-beats-absent per the flicker law). Typed refusals: defer+vector,
  defer-without-data.
- CRASH-SAFE: markers are recovered at open by a BOUNDED prefix listing
  (never a store walk) and the worker resumes in the background — a crash
  can delay a vector, never lose one. A wedged embedder trips a LOUD 60s
  hang guard and the worker moves on (marker retained for retry).
- The honest gauges: getIndexStatus().pendingEmbeds + pendingEmbedCount();
  awaitPendingEmbeds() is the eventual-vector-index BARRIER for callers
  and tests that need searchability before proceeding.
- VFS adopts it everywhere a write path could wait on the embedder:
  writeFile (both branches) and directory creation. Pinned in the
  strongest form: writeFile resolves while the embedder HANGS FOREVER.

Pins: deferred-embedding 5/5 (ack law · stale-beats-absent · crash
recovery across sessions · VFS hung-embedder ack · typed refusals).
Gates: unit 1928/1928 · integration 765 · conformance 27/27.

src/vfs/VirtualFileSystem.ts

@@ -694,6 +694,12 @@ export class VirtualFileSystem implements IVirtualFileSystem {
       await this.brain.update({
         id: existingId,
         data: embeddingData,
+        // MT5: the caller's write acks at durability; the re-embed (a neural
+        // net — it dominated the measured 5.6s p50 per file write) runs on
+        // the background worker and swaps in atomically. Content is readable
+        // and metadata-findable immediately; semantic search converges when
+        // the embed lands (eventual vector index, the documented contract).
+        deferEmbedding: true,
         metadata
       })
 
@@ -729,6 +735,9 @@ export class VirtualFileSystem implements IVirtualFileSystem {
         data: embeddingData,  // Always provide string for embeddings
         type: this.getFileNounType(mimeType),
         subtype: 'vfs-file',  // Standard subtype for VFS file entities (7.30+)
+        // MT5: ack at durability; embedding backgrounds (see the overwrite
+        // branch note above).
+        deferEmbedding: true,
         metadata
       })
 
@@ -1117,6 +1126,9 @@ export class VirtualFileSystem implements IVirtualFileSystem {
         data: path,  // Directory path as string content
         type: NounType.Collection,
         subtype: 'vfs-directory',  // Standard subtype for VFS directory entities (7.30+)
+        // MT5: a directory creation on a write path must not wait on the
+        // embedder either — same ack-at-durability contract as file writes.
+        deferEmbedding: true,
         metadata
       })