Leave the previous board group on hub JoinBoard switch

[Chris0Jeky]

Wave 2B realtime follow-up fix (swarm review). The presence tracker holds one board per connection and silently drops the old board on Join, but the hub never removed the connection from the old SignalR group nor republished the old snapshot, so a JoinBoard without a matching LeaveBoard left ghost presence and stray old-board mutations. JoinBoard now leaves the previous group and publishes the departure snapshot (new TryGetBoard lookup; leaving never requires permission, so revoked users are cleaned too). 4 new tests; BoardsHubSwitch + tracker 6/6, hub integration/concurrency regressions 27/27 pass.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-23T00:42:44.260422Z	e9665e0	PR opened

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[Chris0Jeky]

Estate ops — CI lane (eng supervisor bot) — 2026-09-23

Merge-next slot #3: leave prior board group on hub JoinBoard switch. Eng-ok shape if exact-head CI is green.

Asks

1. Confirm realtime/hub tests cover switch A→B leaves group A (no duplicate membership / no ghost receives).
2. Sequence: after fix(frontend): resolve late-loaded input-assist matches #3269 and Unify CanWrite null-auth stamp between board detail and list #3372; before CONFLICTING fix(board): isolate mutation settlements across logout #3358.
3. Do not bundle logout session isolation (fix(board): isolate mutation settlements across logout #3358) into this PR.

UXQA: hub join chrome usability stays on #2901.

No merge from this bot.

[Chris0Jeky]

Coordinator verification: bug is real on main. Tracker Join already drops the old board mapping (InMemoryBoardPresenceTracker.cs lines 17-25) but BoardsHub.JoinBoard never removed the connection from the old SignalR group nor republished the old snapshot, so a switch without LeaveBoard strands group membership (stray old-board mutations) with stale presence. Fix ordering is right: permission check first (failed switch does not evict), then group-remove plus Leave plus old-snapshot publish before joining the new board; Leave is non-null so the publish call is safe; same-board rejoin and first join are no-ops for the leave path. Tests use the real tracker with mocked SignalR and assert group removal, old snapshot without the user, and new snapshot with the user. Full CI green. Recommend merge once mergeable resolves.