feat(ai-vault-search): expose the index handle a composed reader queries

PR 4's engine reads the index this store owns. One getter lets it compose over
the store's handle instead of opening a second connection to the same file,
and it carries the two rules this PR measured: never hold a read transaction
across an await, and no `.iterate()` outliving its statement. Either pins a
read snapshot, and a checkpoint cannot pass one, so the WAL grows without
bound for as long as it is held (10 MB to 266 MB on the write benchmark).

src/main/ai-vault-search/session-search-file-write.test.ts

@@ -491,6 +491,14 @@ it('writes nothing for an incomplete read and owes the file a whole re-read', ()
   expect(errors).toEqual([])
 })
 
+it('exposes the handle a composed reader queries through', () => {
+  replayTranscriptRead({ messages: userMessages('composedreader', 3) })
+
+  // PR 4's engine reads through this rather than opening a second connection,
+  // so it sees a write the moment the transaction commits.
+  expect(store.connection.prepare('SELECT count(*) AS n FROM messages').get()).toEqual({ n: 3 })
+})
+
 it('closes twice without turning the second call into an error', () => {
   store.close()
   // node:sqlite throws ERR_INVALID_STATE on a second close of one handle, and a

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

@@ -45,6 +45,21 @@ export class SessionSearchStore {
     this.writer = new SessionSearchIndexWriter(this.db)
   }
 
+  /**
+   * The index handle, for a reader composed over this store (PR 4's engine).
+   *
+   * Two rules come with it, both measured in this PR. **Never hold a read
+   * transaction across an `await`**: a checkpoint cannot pass an open read
+   * snapshot, so a paginated read that opened `BEGIN` and yielded between pages
+   * takes the WAL from 10 MB to 266 MB and it does not come back. And **no
+   * `.iterate()` that outlives its statement**, which is the same pin by
+   * another name. Every retrieval a single synchronous statement is the whole
+   * contract.
+   */
+  get connection(): SyncDatabase {
+    return this.db
+  }
+
   setAcceptingWrites(accept: boolean): void {
     this.acceptingWrites = accept
   }