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.

Services/MediaVideoSource.cs

@@ -25,6 +25,8 @@ public sealed class MediaVideoSource : IMediaFrameSource
     private readonly int _height;
     private readonly IFfmpegLocator _locator;
     private readonly IDecodeProcess _process;
+    private readonly IFrameRateProbe? _probe;
+    private readonly Func<TimeSpan, CancellationToken, Task> _delay;
     private readonly CancellationTokenSource _cts = new();
     private IDisposable? _run;
 
@@ -33,13 +35,17 @@ public sealed class MediaVideoSource : IMediaFrameSource
         int width,
         int height,
         IFfmpegLocator locator,
-        IDecodeProcess process)
+        IDecodeProcess process,
+        IFrameRateProbe? frameRateProbe = null,
+        Func<TimeSpan, CancellationToken, Task>? delay = null)
     {
         _path = path;
         _width = width;
         _height = height;
         _locator = locator;
         _process = process;
+        _probe = frameRateProbe;
+        _delay = delay ?? ((t, ct) => Task.Delay(t, ct));
     }
 
     public Task StartAsync(CancellationToken cancellationToken = default)
@@ -70,6 +76,11 @@ public sealed class MediaVideoSource : IMediaFrameSource
     {
         try
         {
+            double? frameRate = null;
+            if (_probe != null)
+                frameRate = await _probe.ProbeAsync(_path, _cts.Token).ConfigureAwait(false);
+            var frameDuration = frameRate is > 0 ? TimeSpan.FromSeconds(1.0 / frameRate.Value) : TimeSpan.Zero;
+
             var ffmpegPath = await _locator.LocateAsync(_cts.Token).ConfigureAwait(false);
             var args =
                 $"-hide_banner -loglevel error -i \"{_path}\" " +
@@ -93,7 +104,11 @@ public sealed class MediaVideoSource : IMediaFrameSource
                 var read = await stream.ReadAsync(buffer, 0, buffer.Length, _cts.Token).ConfigureAwait(false);
                 if (read <= 0) break;
                 foreach (var frame in reader.Feed(buffer.AsSpan(0, read).ToArray()))
+                {
                     FrameAvailable?.Invoke(frame);
+                    if (frameDuration > TimeSpan.Zero)
+                        await _delay(frameDuration, _cts.Token).ConfigureAwait(false);
+                }
             }
         }
         catch (OperationCanceledException)