fix(runtimed): evict re-keyed rooms by Arc pointer, not stale key

[quillaid]

Summary

• Fixes room eviction leak where rooms re-keyed by rekey_ephemeral_room (UUID → file path) were never evicted, leaking fds indefinitely
• The eviction timer now finds the room in the rooms map by Arc pointer comparison instead of the captured notebook_id key, which becomes stale after re-keying

Root cause

When rekey_ephemeral_room moves a room from UUID → file path key, the eviction timer scheduled under the old UUID key fires 30s later and looks up rooms_guard.get(&uuid_key). Since the UUID was removed from the map (replaced by the path key), the lookup returns None and eviction is skipped. No new eviction timer is ever scheduled under the new path key. The room lives forever with its kernel, runtime agent socket, file watcher, and autosave debouncer.

Evidence from production daemon (7h uptime)

Metric	Count
Rooms created	237
Eviction timers scheduled	248
Eviction skipped (stale key after re-key)	163
Actually evicted	81
Cancelled (peers reconnected)	4
Zombie rooms (0 peers, idle kernel)	144
Daemon fds	695/1024 (68%)

Fix

The eviction timer closure now iterates the rooms map to find the entry matching the captured Arc<NotebookRoom> by pointer. The Arc is stable across re-keying since rekey_ephemeral_room moves the same Arc to the new key. This is O(n) in room count but runs at most once per room disconnect — negligible vs the existing O(doc_size) teardown work.

Test plan

• All runtimed tests pass (25/25)
• Lint clean, tokio mutex lint passes
• Deploy locally, verify zombie rooms are evicted after 30s idle timeout
• Monitor fd count — should stabilize instead of monotonically increasing