TASK 21 Increment B: FFmpeg rawvideo video decoder -> VideoFrame

Adds the codec-agnostic decoder half of the media source. Spawns ffmpeg
with -f rawvideo -pix_fmt bgra (reusing the already-shipped ffmpeg via
IFfmpegLocator) and drains the raw BGRA stdout pipe into VideoFrames.

- Services/RawVideoFrameReader.cs: pure rawvideo BGRA stream -> frames
  (partial reads kept across Feed; no ffmpeg needed to test)
- Services/IDecodeProcess.cs + FfmpegDecodeProcess.cs: binary-stdout
  subprocess seam, mirror of the encoder's IEncoderProcess
- Services/MediaVideoSource.cs: owns the decode, raises FrameReady/Completed
- MediaVideoSourceTests: 3 pure reader + 1 integration (fake decode
  process through the real source loop, frames in order)

Reference (external scan): ffmpeg rawvideo pipe decode is the canonical
codec-agnostic frame feeds pattern (ffmpeg docs -f rawvideo; how OBS/media
pipelines push frames to a compositor). Verified: 4/4 tests, build 0 warnings.
Native-FPS pacing + resolver/compositor wiring are the next slice.

Services/FfmpegDecodeProcess.cs

@@ -0,0 +1,36 @@
+using System.Diagnostics;
+using System.IO;
+
+namespace ytLive.Services;
+
+/// <summary>
+/// The real <see cref="IDecodeProcess"/>: a <see cref="Process"/> with stdout
+/// redirected as raw bytes for reading <c>rawvideo</c> frames. Constructed by
+/// <see cref="MediaVideoSource"/> for the media-file decoder subprocess.
+/// </summary>
+public sealed class FfmpegDecodeProcess : IDecodeProcess
+{
+    private readonly Process _process;
+
+    public FfmpegDecodeProcess() => _process = new Process { EnableRaisingEvents = true };
+
+    public void Start(ProcessStartInfo startInfo)
+    {
+        _process.StartInfo = startInfo;
+        _process.Start();
+    }
+
+    public Stream StandardOutput => _process.StandardOutput.BaseStream;
+
+    public bool HasExited => _process.HasExited;
+    public int ExitCode => _process.ExitCode;
+    public void Kill()
+    {
+        if (!_process.HasExited) _process.Kill();
+    }
+
+    public Task WaitForExitAsync(CancellationToken cancellationToken = default)
+        => _process.WaitForExitAsync(cancellationToken);
+
+    public void Dispose() => _process.Dispose();
+}