Transitions: fade starts from the outgoing picture (was an instant cut)

The transition started with no from-frame, so BlendFrame returned the new scene at once.
Start now requests a capture and the frame pump supplies its last output on the next tick.
Test: TransitionFadeTests. Docs: ai.md, TASKS.md.

Not pushed.

Services/TransitionService.cs

@@ -45,12 +45,23 @@ public sealed class TransitionService
     /// </summary>
     public VideoFrame? FromFrame { get; private set; }
 
+    /// <summary>True when a transition started without its "from" picture; the frame pump
+    /// supplies the outgoing program frame on its next tick via <see cref="SupplyFromFrame"/>.</summary>
+    public bool CaptureRequested { get; private set; }
+
+    public void SupplyFromFrame(VideoFrame frame)
+    {
+        FromFrame = frame;
+        CaptureRequested = false;
+    }
+
     /// <summary>Begin a transition. Caller should capture FromFrame from the current LiveScene render.</summary>
     public void Start(TransitionType type, double durationMs, VideoFrame? fromFrame)
     {
         CurrentType = type;
         DurationMs = durationMs > 0 ? durationMs : DefaultDurationMs;
         FromFrame = fromFrame;
+        CaptureRequested = fromFrame == null && type != TransitionType.Cut;
         ElapsedMs = 0;
         Active = true;