fix(stream): keep recovery notifications and stop on the dispatcher

The FFmpeg ProcessFailed event arrives off-thread. Start recovery on the UI
dispatcher, publish retry status there, and route StopStream back to the
dispatcher before changing bound state or alert collections. This prevents a
recovery exception from becoming an unhandled WPF CollectionView crash.

AttentionAlertStackTests covers worker-thread recovery reporting and stop.
616 tests green, clean build with 0 warnings.

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -406,6 +406,13 @@ public partial class MainViewModel : ViewModelBase
 
     private async void StopStream()
     {
+        var dispatcher = Application.Current?.Dispatcher;
+        if (dispatcher != null && !dispatcher.CheckAccess())
+        {
+            _ = dispatcher.BeginInvoke(new Action(StopStream));
+            return;
+        }
+
         CancelStreamRecovery();   // a stop wins over a relaunch in flight
         StopStepDownWatch();
         StopClipRing();