Let a status flap fence the feed instead of throwing it away

MAC3-R1 from the fix verification: refreshCachesForEffectiveContext purged
on every effective-context move, so the cache the Subscriptions fix meant
to keep was already empty when its keep-path ran. A move under an unchanged
selection now fences the feed cache non-destructively; only a genuine
selection move still purges. Also settles the destination loader's
observer callback onto the main actor without a capture-of-var warning.

Yattee/Views/Components/SubscriptionDestinationMenu.swift

@@ -73,7 +73,7 @@ final class SubscriptionDestinationMembershipLoader {
         for name: Notification.Name in [.accountCredentialDidChange, .accountsDidChange, .instancesDidChange] {
             observers.append(
                 NotificationCenter.default.addObserver(forName: name, object: nil, queue: .main) { [weak self] _ in
-                    Task { @MainActor in self?.reload() }
+                    MainActor.assumeIsolated { self?.reload() }
                 }
             )
         }