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).

Services/IMediaFrameSource.cs

@@ -10,10 +10,15 @@ public interface IMediaFrameSource : IDisposable
 {
     string Key { get; }
 
+    /// <summary>When true, the decoder restarts from the beginning on natural EOF
+    /// instead of raising <see cref="Completed"/> (used for BRB/starting-soon loops).</summary>
+    bool Looping { get; set; }
+
     /// <summary>Raised (on a worker thread) for each decoded frame.</summary>
     event Action<VideoFrame>? FrameAvailable;
 
-    /// <summary>Raised when the file finishes decoding (natural EOF).</summary>
+    /// <summary>Raised when the file finishes decoding (natural EOF) — not raised
+    /// while <see cref="Looping"/> is true.</summary>
     event Action? Completed;
 
     Task StartAsync(CancellationToken cancellationToken = default);

Services/MediaVideoSource.cs

@@ -8,8 +8,9 @@ namespace ytLive.Services;
 /// TASK 21: decodes a local media file (any format ffmpeg handles) into
 /// <see cref="VideoFrame"/>s for a MediaSource. Implements
 /// <see cref="IMediaFrameSource"/>: raises <see cref="FrameAvailable"/> per
-/// frame and <see cref="IMediaFrameSource.Completed"/> on natural EOF. Spawns
-/// ffmpeg with <c>-f rawvideo -pix_fmt bgra</c> and drains the raw BGRA pipe via
+/// frame and <see cref="IMediaFrameSource.Completed"/> on natural EOF (unless
+/// <see cref="IMediaFrameSource.Looping"/>). Spawns ffmpeg with
+/// <c>-f rawvideo -pix_fmt bgra</c> and drains the raw BGRA pipe via
 /// <see cref="RawVideoFrameReader"/>. The decoder process and emitters are seams
 /// so tests run without a real codec or audio device.
 /// </summary>
@@ -20,22 +21,25 @@ public sealed class MediaVideoSource : IMediaFrameSource
 
     public string Key => _path;
 
+    public bool Looping { get; set; }
+
     private readonly string _path;
     private readonly int _width;
     private readonly int _height;
     private readonly IFfmpegLocator _locator;
-    private readonly IDecodeProcess _process;
+    private readonly Func<IDecodeProcess> _processFactory;
     private readonly IFrameRateProbe? _probe;
     private readonly Func<TimeSpan, CancellationToken, Task> _delay;
     private readonly CancellationTokenSource _cts = new();
     private IDisposable? _run;
+    private IDecodeProcess? _active;
 
     public MediaVideoSource(
         string path,
         int width,
         int height,
         IFfmpegLocator locator,
-        IDecodeProcess process,
+        Func<IDecodeProcess> processFactory,
         IFrameRateProbe? frameRateProbe = null,
         Func<TimeSpan, CancellationToken, Task>? delay = null)
     {
@@ -43,7 +47,7 @@ public sealed class MediaVideoSource : IMediaFrameSource
         _width = width;
         _height = height;
         _locator = locator;
-        _process = process;
+        _processFactory = processFactory;
         _probe = frameRateProbe;
         _delay = delay ?? ((t, ct) => Task.Delay(t, ct));
     }
@@ -67,7 +71,7 @@ public sealed class MediaVideoSource : IMediaFrameSource
     {
         _cts.Cancel();
         _cts.Dispose();
-        _process.Dispose();
+        _active?.Dispose();
     }
 
     private void StopInternal() => KillProcess();
@@ -94,22 +98,32 @@ public sealed class MediaVideoSource : IMediaFrameSource
                 RedirectStandardError = true,
                 CreateNoWindow = true,
             };
-            _process.Start(startInfo);
 
-            var reader = new RawVideoFrameReader(_width, _height);
-            var buffer = new byte[65536];
-            var stream = _process.StandardOutput;
-            while (!_cts.IsCancellationRequested && !_process.HasExited)
+            do
             {
-                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()))
+                using (var process = _processFactory())
                 {
-                    FrameAvailable?.Invoke(frame);
-                    if (frameDuration > TimeSpan.Zero)
-                        await _delay(frameDuration, _cts.Token).ConfigureAwait(false);
+                    _active = process;
+                    process.Start(startInfo);
+
+                    var reader = new RawVideoFrameReader(_width, _height);
+                    var buffer = new byte[65536];
+                    var stream = process.StandardOutput;
+                    while (!_cts.IsCancellationRequested && !process.HasExited)
+                    {
+                        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);
+                        }
+                    }
                 }
+                _active = null;
             }
+            while (Looping && !_cts.IsCancellationRequested);
         }
         catch (OperationCanceledException)
         {
@@ -128,6 +142,6 @@ public sealed class MediaVideoSource : IMediaFrameSource
 
     private void KillProcess()
     {
-        try { _process.Kill(); } catch { /* already exited */ }
+        try { _active?.Kill(); } catch { /* already exited */ }
     }
 }