## Why
Archiving a thread also archives its spawned descendants, but unarchiving it previously restored only the parent.
## What changed
- Restore archived spawned descendants when handling `thread/unarchive`, including descendants archived independently of the parent. Skip active descendants and continue past descendant restoration failures; fall back to restoring only the parent if descendant discovery fails.
- Emit `thread/unarchived` for each successfully restored thread. Hold the thread-list mutation permit through archive and unarchive responses and notifications to preserve ordering with other list mutations.
- Add `ThreadStore::unarchive_threads` and share rollout discovery across the subtree in the local store.
## Testing
Extend coverage for subtree restoration, independently archived children, descendant restoration failures, and failed descendant discovery. Store tests cover skipping active descendants and restoring archived descendants without hydrating their metadata.
GitOrigin-RevId: c54ca099c92bcb2df5e595fa252d7fad7c018760
Eric Traut
6ba3a84680