Confirm resumed thread subscriptions

[twistedgrim]

Summary

• Add a synchronized stream item after a resumed thread subscription has attached its live buffer.
• Keep a warm cached thread synchronizing through reconnect until that item arrives.

Reproduction

1. Start with a cached thread detail snapshot and its persisted snapshot sequence.
2. Reconnect and subscribe using the cached sequence as the after-sequence cursor.
3. Make the server replay empty because the cache is already current.

Before this change, the client could only infer completion from the transport state; no stream frame proved that catch-up and the live buffer were attached. This PR makes that boundary explicit. The server test exercises the empty-replay case, and the client-state test verifies that a cached thread stays synchronizing until the confirmation arrives.

Validation

• vp test apps/server/src/server.test.ts packages/client-runtime/src/state/threads-sync.test.ts
• vp check
• vp run typecheck