TASK 21 slice 2b: native-FPS pacing in MediaVideoSource (probe -> delay seam)

- MediaVideoSource takes optional IFrameRateProbe? + Func<TimeSpan,CancellationToken,Task>? delay
  seams (default Task.Delay); probes FPS once in RunAsync, delays by 1/fps after each
  emitted frame. No probe/unknown fps -> no pacing (ffmpeg pipe backpressure throttles).
- Test: MediaVideoSource_PacesFramesByProbedFps (fake probe returns 1000fps + recording
  delay; one delay per frame ~= 1ms). Media tests 5/5, build 0 warnings.

Derivative reference: per-frame delay pacing of decoded output, standard in media playback.

ytLive.Tests/MediaVideoSourceTests.cs

@@ -90,6 +90,41 @@ public class MediaVideoSourceTests
         public void Dispose() => _stream.Dispose();
     }
 
+    [Fact]
+    public async Task MediaVideoSource_PacesFramesByProbedFps()
+    {
+        const int w = 2, h = 1; // frame bytes = 8
+        var payload = new byte[16]; // two frames
+        var delays = new List<TimeSpan>();
+
+        using var source = new MediaVideoSource(
+            "clip.mp4", w, h,
+            new FakeLocator(),
+            new FakeDecodeProcess(payload),
+            frameRateProbe: new FakeFrameRateProbe(1000.0),
+            delay: (_d, _) => { delays.Add(_d); return Task.CompletedTask; });
+
+        var completed = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+        source.Completed += () => completed.TrySetResult(true);
+
+        await source.StartAsync();
+        await completed.Task.WaitAsync(TimeSpan.FromSeconds(5));
+
+        // One pacing delay per emitted frame, each = 1/1000s.
+        Assert.Equal(2, delays.Count);
+        foreach (var d in delays)
+            Assert.True(Math.Abs((d - TimeSpan.FromMilliseconds(1)).TotalMilliseconds) < 0.001,
+                $"expected ~1ms, got {d.TotalMilliseconds}ms");
+    }
+
+    private sealed class FakeFrameRateProbe : IFrameRateProbe
+    {
+        private readonly double? _fps;
+        public FakeFrameRateProbe(double? fps) => _fps = fps;
+        public Task<double?> ProbeAsync(string path, CancellationToken ct = default)
+            => Task.FromResult(_fps);
+    }
+
     private sealed class FakeLocator : IFfmpegLocator
     {
         public Task<string> LocateAsync(CancellationToken ct = default) => Task.FromResult("ffmpeg.exe");