* Move UNUserNotificationCenter remove calls off main thread
removeDeliveredNotifications(withIdentifiers:) and
removePendingNotificationRequests(withIdentifiers:) perform synchronous XPC
to usernoted. When usernoted is slow or overwhelmed, this blocks the main
thread indefinitely, freezing the entire UI (confirmed via sample showing
__NSXPCCONNECTION_IS_WAITING_FOR_A_SYNCHRONOUS_REPLY__ for the full sample
duration on both cmux and cmux NIGHTLY).
Add extension methods on UNUserNotificationCenter that dispatch these calls
to a background queue. All 13 call sites in TerminalNotificationStore are
fire-and-forget (void, no data flows back), so moving them off-main is safe.
The @Published property mutations and dock badge updates remain on @MainActor.
* Use dedicated serial queue for notification removal dispatch
Replaces DispatchQueue.global(qos: .utility) with a private static serial
queue. If usernoted stalls, concurrent dispatches to the global pool could
exhaust threads. A dedicated queue caps concurrency at 1.
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