test(ai-vault-search): make the retirement cap test prove its ordering

Removing the newest-first sort from a cycle's retirement scope failed nothing:
the fixture wrote its transcripts oldest first and the sweep indexed them
newest first, so the table's own row order already put the oldest last and an
unsorted slice happened to reach the same answer. The oldest file is now
indexed on its own first, which makes it the earliest row as well as the oldest
file, and the two orders disagree.

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

@@ -950,22 +950,32 @@ it('sweeps on its cadence without anyone asking', async () => {
 // recent enough to be in the slice, and the rest are the next sweep's to reach.
 it('proves deletions for the newest rows it holds, and leaves the tail to a sweep', async () => {
   const total = 530
-  const paths: string[] = []
-  for (let index = 0; index < total; index++) {
+  const oldest = transcriptPath('00000000-bbbb-4ccc-8ddd-eeeeeeeeeeee')
+  await writeClaudeTranscript(
+    oldest,
+    ['the oldest session'],
+    '00000000-bbbb-4ccc-8ddd-eeeeeeeeeeee'
+  )
+  const longAgo = new Date(Date.now() - total * 60_000)
+  await utimes(oldest, longAgo, longAgo)
+  // Indexed on its own first, so it is the earliest row in the table as well as
+  // the oldest file. A slice that trusted the table's own order rather than the
+  // mtime would take it, and take it first.
+  await newIndexer().start()
+
+  for (let index = 1; index < total; index++) {
     const session = `0000${String(index).padStart(4, '0')}-bbbb-4ccc-8ddd-eeeeeeeeeeee`
     const path = transcriptPath(session)
     await writeClaudeTranscript(path, [`capped session ${index}`], session)
-    // Oldest first, so the file deleted below is at the far end of the slice.
     const at = new Date(Date.now() - (total - index) * 60_000)
     await utimes(path, at, at)
-    paths.push(path)
   }
-  await newIndexer().start()
+  await indexer?.reconcile({ full: true })
   expect(indexedSessionCount()).toBe(total)
 
   // Older than the cap reaches: 530 rows, twelve of them rediscovered by the
   // cycle, leaves 518 undiscovered against a cap of 512.
-  await rm(paths[0] ?? '')
+  await rm(oldest)
   await nextCycle()
   expect(indexedSessionCount()).toBe(total)