TASK 4 ship step 5: live frame pipeline — FramePump paces the active scene's composite into the encoder, ScreenCaptureManager.GetLatestFrame, MainViewModel resolver/option-builder/pump wiring, FramePumpTests (7) + GetLatestFrame test — 147 tests passing, 0 warnings

Services/ScreenCaptureManager.cs

@@ -156,6 +156,15 @@ public sealed class ScreenCaptureManager : IDisposable
         toStop.PreviewBitmap = null;
     }
 
+    /// <summary>The most recent frame for a capture key, or null before the first
+    /// frame arrives (or if the key has no session). The live compositor reads
+    /// the backdrop from here.</summary>
+    public VideoFrame? GetLatestFrame(string key)
+    {
+        lock (_gate)
+            return _sessions.TryGetValue(key, out var session) ? session.LatestFrame : null;
+    }
+
     public void Dispose()
     {
         List<CaptureSession> sessions;