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).

ytLive.Tests/MediaVideoSourceManagerTests.cs

@@ -20,6 +20,7 @@ public class MediaVideoSourceManagerTests
         private readonly List<string>? _stopped;
 
         public string Key { get; }
+        public bool Looping { get; set; }
         public event Action<VideoFrame>? FrameAvailable;
         public event Action? Completed;
 
@@ -50,6 +51,7 @@ public class MediaVideoSourceManagerTests
     private sealed class FailingMediaSource : IMediaFrameSource
     {
         public string Key { get; }
+        public bool Looping { get; set; }
         public event Action<VideoFrame>? FrameAvailable;
         public event Action? Completed;
         public FailingMediaSource(string key) => Key = key;