fix(ai-vault-search): clear a file's stale mark once the backfill indexes it

List scans queue every not-yet-indexed file as stale while the backfill runs;
nothing cleared the mark when the backfill reached the file, so the first
search after a backfill re-parsed the whole queue (8.4 s on 4,000 sessions).

src/main/ai-vault-search/session-search-store.test.ts

@@ -149,6 +149,21 @@ describe('SessionSearchStore', () => {
     expect(store.coverage().filesPending).toBe(0)
   })
 
+  it('drops a stale mark once the backfill indexes the file', async () => {
+    const root = await makeTempDir()
+    const path = join(root, `${SESSION_ID}.jsonl`)
+    await writeFile(path, `${userRecord(0, 'queued behind the backfill')}\n`)
+    registerSessionSearchIndexSink(null)
+    await parse(path)
+    registerSessionSearchIndexSink(store)
+    await parse(path)
+    expect(store.coverage().filesPending).toBe(1)
+
+    await withSessionSearchIndexRequired(() => parse(path))
+    expect(store.coverage()).toMatchObject({ sessionsIndexed: 1, filesPending: 0 })
+    expect(store.takeStale()).toEqual([])
+  })
+
   it('repairs a typo from the index vocabulary', async () => {
     const root = await makeTempDir()
     const path = join(root, `${SESSION_ID}.jsonl`)

src/main/ai-vault-search/session-search-store.ts

@@ -58,6 +58,9 @@ export class SessionSearchStore implements SessionSearchIndexSink {
   apply(update: SessionSearchIndexUpdate): void {
     try {
       this.writer.apply(update)
+      // Why: list scans queue every file the backfill has not reached yet; once
+      // it lands, a later search must not re-parse the whole queue (8 s live).
+      this.stale.delete(update.candidate.file.path)
       this.lastIndexedAt = new Date().toISOString()
     } catch (error) {
       this.applyFailures += 1