## What changed
- Add `ThreadManager::with_thread_id_generator` to configure ID allocation for new root, child, and forked threads.
- Keep the standard UUIDv7 generator as the default and preserve stored IDs when resuming threads.
- Add `ThreadId::from_u128` for constructing identifiers from UUID-compatible values.
## Testing
- Cover default allocation, custom IDs across roots, child agents, and forks, and ID preservation on resume.
GitOrigin-RevId: 6c38c862e30a50bad944a994bcbccf7d1e7ada24