TASK 21 slice 3: loop control in MediaVideoSource (process factory + Loop flag)

- IMediaFrameSource gains bool Looping.
- MediaVideoSource ctor takes Func<IDecodeProcess> processFactory instead of a
  single IDecodeProcess: a System.Diagnostics.Process can't be re-Start()ed, so
  each loop pass creates a fresh decoder. Decode wrapped in do-while(Looping):
  restart on natural EOF instead of raising Completed.
- Production wiring (MainViewModel media factory): passes the process factory
  AND FfmpegFrameRateProbe -- closes the slice-2b gap where production had no
  probe and therefore no pacing.
- Tests: loop test (single frame re-emits across passes, Completed only when
  loop cleared); fakes updated for the new interface member. Media tests 12/12,
  build 0 warnings.

Wiring Source.MediaIsLooping into the flag needs a manager-level per-path loop
provider -> lands with the UI-picker (acquisition) slice.

Derivative reference: looping media by restarting decode on EOF, standard in
playback/overlay tooling (OBS media source repeat).

Services/IMediaFrameSource.cs

@@ -10,10 +10,15 @@ public interface IMediaFrameSource : IDisposable
 {
     string Key { get; }
 
+    /// <summary>When true, the decoder restarts from the beginning on natural EOF
+    /// instead of raising <see cref="Completed"/> (used for BRB/starting-soon loops).</summary>
+    bool Looping { get; set; }
+
     /// <summary>Raised (on a worker thread) for each decoded frame.</summary>
     event Action<VideoFrame>? FrameAvailable;
 
-    /// <summary>Raised when the file finishes decoding (natural EOF).</summary>
+    /// <summary>Raised when the file finishes decoding (natural EOF) — not raised
+    /// while <see cref="Looping"/> is true.</summary>
     event Action? Completed;
 
     Task StartAsync(CancellationToken cancellationToken = default);