## Why
A thread ID alone does not identify the backend that owns a session. Resume and fork selections must retain that ownership across connection replacement so they cannot operate on another backend, even when thread IDs match.
## What changed
- Add a process-local `BackendId` to app-server sessions and session targets, preserving it across reconnects and auxiliary picker connections.
- Reject selections from a different backend before changing session state or sending resume, fork, or unarchive requests.
- Disable local-daemon fallback to an embedded server for auxiliary pickers and after selecting a session, while retaining fallback for fresh startup.
## Testing
Add regression coverage for foreign selections with matching thread IDs, startup resume and fork rejection before requests, shared ownership across picker connections, and fallback eligibility.
GitOrigin-RevId: 05a3ca0a901a8dc24337d130548aa2c048b4d586
Benjamin Carlsson
440312f113