## Why
Callers need to associate host-owned state with a thread before Core starts it.
## What changed
- Add `ThreadManager::reserve_thread_id` and allow new threads to start with the reserved ID. Reject reserved IDs when resuming an existing thread.
- Let thread stores stage metadata for a reserved ID. The local store merges staged values into the first successful metadata update and then removes them.
- Clear staged metadata when an unmaterialized thread shuts down or is discarded, and require the state database for staging.
## Testing
- Cover reserved ID generation and resume validation.
- Cover metadata persistence, merging, rollout-compatible updates, cleanup, and invalid staging requests.
GitOrigin-RevId: 3f9bc48f0f7764011cc90ea01349e497dda0a9e3
Anton Panasenko
fcdae21073