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;

ytLive.Tests/MediaVideoSourceTests.cs

@@ -62,7 +62,7 @@ public class MediaVideoSourceTests
         using var source = new MediaVideoSource(
             "clip.mp4", w, h,
             new FakeLocator(),
-            new FakeDecodeProcess(payload));
+            () => new FakeDecodeProcess(payload));
 
         var seen = new List<byte[]>();
         var completed = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
@@ -100,7 +100,7 @@ public class MediaVideoSourceTests
         using var source = new MediaVideoSource(
             "clip.mp4", w, h,
             new FakeLocator(),
-            new FakeDecodeProcess(payload),
+            () => new FakeDecodeProcess(payload),
             frameRateProbe: new FakeFrameRateProbe(1000.0),
             delay: (_d, _) => { delays.Add(_d); return Task.CompletedTask; });
 
@@ -117,6 +117,37 @@ public class MediaVideoSourceTests
                 $"expected ~1ms, got {d.TotalMilliseconds}ms");
     }
 
+    [Fact]
+    public async Task MediaVideoSource_LoopsUntilLoopDisabled()
+    {
+        const int w = 1, h = 1; // frame bytes = 4
+        var payload = new byte[4];
+        var passCount = 0;
+        var frames = 0;
+        var completed = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        using var source = new MediaVideoSource(
+            "clip.mp4", w, h,
+            new FakeLocator(),
+            () => { passCount++; return new FakeDecodeProcess(payload); });
+        source.Looping = true;
+        source.Completed += () => completed.TrySetResult(true);
+        source.FrameAvailable += _ =>
+        {
+            frames++;
+            if (frames >= 3) source.Looping = false;
+        };
+
+        await source.StartAsync();
+        await completed.Task.WaitAsync(TimeSpan.FromSeconds(5));
+        await source.StopAsync();
+
+        // The single frame re-emits across passes (restart) and Completed only
+        // fires once Looping is cleared.
+        Assert.True(frames >= 3, $"expected the single frame to re-emit across passes, got {frames}");
+        Assert.True(passCount >= 3, "Looping should have restarted the decode process");
+    }
+
     private sealed class FakeFrameRateProbe : IFrameRateProbe
     {
         private readonly double? _fps;