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).

ViewModels/MainViewModel.cs

@@ -285,7 +285,9 @@ public partial class MainViewModel : ViewModelBase
         _mediaManager = new MediaVideoSourceManager(
             path => new MediaVideoSource(
                 path, MediaDecodeWidth, MediaDecodeHeight,
-                new FfmpegLocator(), new FfmpegDecodeProcess()),
+                new FfmpegLocator(), () => new FfmpegDecodeProcess(),
+                frameRateProbe: new FfmpegFrameRateProbe(
+                    new FfmpegLocator(), () => new FfmpegDecodeProcess())),
             System.Windows.Application.Current?.Dispatcher);
         _mediaManager.PreviewBitmapChanged += OnMediaPreviewBitmapChanged;
         _mediaManager.MediaFailed += OnMediaFailed;