CD-015: rewrite NotificationOutboxRepository.InsertIfNotExistsAsync as raw-SQL
IF NOT EXISTS … INSERT with SqlException 2601/2627 catch, ending the
at-least-once livelock on the site→central notification handoff.
DCL-018/019/020/021/022: add _subscribesInFlight guard so concurrent
same-tag subscribes don't orphan an adapter handle; delete the latent
dead _subscriptionHandles dictionary; stop double-counting
_totalSubscribed when an unresolved tag is promoted via another instance;
release adapter handles on mid-flight unsubscribe; gate the
tag-resolution retry timer with IsTimerActive so subscribe bursts don't
reset it into starvation.
SR-020: add _terminatingActorsByName shadow so a third deploy arriving
during a pending redeploy doesn't crash on InvalidActorNameException —
displaced senders get a Failed/superseded response and the latest
command wins on Terminated.
SR-024: split OperationTrackingStore reads from writes (fresh
SqliteConnection per GetStatusAsync) so long writes don't block status
queries; rewrite Dispose to drop the sync-over-async bridge that could
deadlock on a non-reentrant SyncContext; Interlocked.Exchange makes the
dispose-once flag race-safe across both paths.
Joseph Doherty