FencingManager heartbeat orphaned on expired token takeover

[pallyoung]

Problem

When a new client takes over an expired token in FencingManager.requestControl(), it deletes the expired token from this.tokens but does not clean up the old clientId entry from this.heartbeats.

Code path

packages/server/src/ws/fencing.ts, line 83-85:

if (now > existing.expiresAt) {
  // Token expired, take over
  this.tokens.delete(workspaceId);
  // ← missing: this.heartbeats.delete(existing.clientId);
}

Scenario

1. Tab A gets controller → heartbeat recorded
2. Tab A disconnects (crash, network, etc.) → release() never called
3. Token expires (30s)
4. Tab B calls requestControl(), sees expired token, deletes it
5. Tab A's heartbeat entry in heartbeats is now orphaned — never cleaned up

Impact

Minor memory leak. Each orphaned entry is one Map entry (~tens of bytes). Not functionally impactful.

Suggested Fix

Add this.heartbeats.delete(existing.clientId) after deleting the expired token on line 85.