Transitions in edit mode and on output: program transitions run while recording or streaming; the preview animates the outgoing picture (fade/move fade, wipe shrinks) in every mode

Test: PreviewTransitionTests (with the transition, wipe, and brand logo tests).

ViewModels/MainViewModel.Scenes.cs

@@ -115,14 +115,20 @@ public partial class MainViewModel
     public void TransitionToScene(Scene scene)
     {
         if (scene.IsHidden || scene == StagedScene) return;
+        // The preview animates the outgoing picture before the switch, in every mode.
+        PreviewTransition?.Invoke(SelectedTransitionType, TransitionDurationMs);
         StagedScene = scene;
-        if (IsLive)
+        // The program output transitions whenever it is running: recording or streaming.
+        if (IsLive || IsRecording)
         {
             LiveScene = scene;
             _transition.Start(SelectedTransitionType, TransitionDurationMs, null);
         }
     }
 
+    /// <summary>Raised before a scene switch so the preview can animate the old picture away.</summary>
+    public event Action<TransitionType, double>? PreviewTransition;
+
     /// <summary>
     /// Apply the current transition type to the next StagedScene change while live.
     /// Called from XAML when user picks a transition from the dropdown.