feat(media): play the clip's soundtrack and share one decode pipe

A video file was picture-only: the decode pipe passed `-an`, so a starting-soon
or BRB clip with music played silent everywhere. Adds sound to every clip and
collapses the two decoders into one.

- `Services/VideoClipPipe.cs`: the shared pipe behind `MediaVideoSource` (TASK 21)
  and `AlertClipDecoder` (TASK 47) — video (`rawvideo bgra`, scaled) + audio
  (`f32le -ac 2 -ar 48000`, 50 ms chunks, byte-carry across reads), both paced to
  real time, loop restarts both so picture and sound stay together. Alert
  behaviour is unchanged.
- `MediaVideoSourceManager` applies the layer's Level/Mute per session and raises
  `AudioAvailable`; `MainViewModel` feeds it to the new
  `AudioMixer.EnqueueDesktopMediaAudio`, which joins the DESKTOP/GAME ring so the
  desktop fader and the mic auto-ducker apply exactly as they do for game audio
  and TRAX (creator ruling 2026-11-05: audio joins the group).
- Per-layer `MediaMuted` + existing `MediaVolume` surfaced as Mute/Level in the
  properties panel; `MediaMuted` persisted (ALTER TABLE guard + SELECT/INSERT).
- Level/mute are per SESSION because one decoder is shared per file path — the
  last layer touched wins; one decoder per layer would play the file twice.

Test: `MediaSourceAudioTests` — the pipe emits 50 ms chunks at 48k stereo, the
manager scales by level and drops when muted, and a media layer's sound moves the
desktop meter. Alert/decode suites unchanged and green.

Docs: task-21 closed on item 10, the "silent soundtrack" facts corrected in
frame-pipeline/output/overlays, HANDOFF rewritten. verify.sh green (529 passed;
the two documented pre-existing failures only).

Services/MediaVideoSource.cs

@@ -1,38 +1,32 @@
-using System.Diagnostics;
-using System.IO;
+using ytLive.Services.Audio;
 using ytLive.Services.Encoder;
 
 namespace ytLive.Services;
 
 /// <summary>
 /// 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 (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.
+/// <see cref="VideoFrame"/>s for a MediaSource. This is the <see cref="IMediaFrameSource"/>
+/// shape over the shared <see cref="VideoClipPipe"/> — the same pipe an alert clip uses
+/// (TASK 47), so the two features cannot drift apart again in how they decode.
+/// <para>Video AND the file's own soundtrack are decoded. The manager scales the audio by
+/// the layer's level/mute and hands it to the mixer's desktop group.</para>
 /// </summary>
 public sealed class MediaVideoSource : IMediaFrameSource
 {
     public event Action<VideoFrame>? FrameAvailable;
+    public event Action<AudioSample>? AudioAvailable;
     public event Action? Completed;
 
     public string Key => _path;
 
-    public bool Looping { get; set; }
+    public bool Looping
+    {
+        get => _clip.Looping;
+        set => _clip.Looping = value;
+    }
 
     private readonly string _path;
-    private readonly int _width;
-    private readonly int _height;
-    private readonly IFfmpegLocator _locator;
-    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;
+    private readonly VideoClipPipe _clip;
 
     public MediaVideoSource(
         string path,
@@ -44,104 +38,16 @@ public sealed class MediaVideoSource : IMediaFrameSource
         Func<TimeSpan, CancellationToken, Task>? delay = null)
     {
         _path = path;
-        _width = width;
-        _height = height;
-        _locator = locator;
-        _processFactory = processFactory;
-        _probe = frameRateProbe;
-        _delay = delay ?? ((t, ct) => Task.Delay(t, ct));
+        _clip = new VideoClipPipe(
+            path, width, height, locator, processFactory, frameRateProbe, delay, audio: true);
+        _clip.FrameAvailable += frame => FrameAvailable?.Invoke(frame);
+        _clip.AudioAvailable += sample => AudioAvailable?.Invoke(sample);
+        _clip.Completed += () => Completed?.Invoke();
     }
 
-    public Task StartAsync(CancellationToken cancellationToken = default)
-    {
-        if (_run != null) return Task.CompletedTask;
-        _run = _cts.Token.Register(StopInternal);
-        _ = Task.Run(RunAsync, CancellationToken.None);
-        return Task.CompletedTask;
-    }
+    public Task StartAsync(CancellationToken cancellationToken = default) => _clip.StartAsync(cancellationToken);
 
-    public Task StopAsync()
-    {
-        _cts.Cancel();
-        _run = null;
-        return Task.CompletedTask;
-    }
+    public Task StopAsync() => _clip.StopAsync();
 
-    public void Dispose()
-    {
-        _cts.Cancel();
-        _cts.Dispose();
-        _active?.Dispose();
-    }
-
-    private void StopInternal() => KillProcess();
-
-    private async Task RunAsync()
-    {
-        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}\" " +
-                $"-f rawvideo -pix_fmt bgra -vf scale={_width}:{_height} -an -";
-            var startInfo = new ProcessStartInfo
-            {
-                FileName = ffmpegPath,
-                Arguments = args,
-                UseShellExecute = false,
-                RedirectStandardOutput = true,
-                RedirectStandardError = true,
-                CreateNoWindow = true,
-            };
-
-            do
-            {
-                using (var process = _processFactory())
-                {
-                    _active = process;
-                    process.Start(startInfo);
-
-                    var reader = new RawVideoFrameReader(_width, _height);
-                    var buffer = new byte[65536];
-                    var stream = process.StandardOutput;
-                    while (!_cts.IsCancellationRequested && !process.HasExited)
-                    {
-                        var read = await stream.ReadAsync(buffer, 0, buffer.Length, _cts.Token).ConfigureAwait(false);
-                        if (read <= 0) break;
-                        foreach (var frame in reader.Feed(buffer.AsSpan(0, read).ToArray()))
-                        {
-                            FrameAvailable?.Invoke(frame);
-                            if (frameDuration > TimeSpan.Zero)
-                                await _delay(frameDuration, _cts.Token).ConfigureAwait(false);
-                        }
-                    }
-                }
-                _active = null;
-            }
-            while (Looping && !_cts.IsCancellationRequested);
-        }
-        catch (OperationCanceledException)
-        {
-        }
-        catch (Exception ex)
-        {
-            Fail($"Media decode failed: {ex.Message}");
-            return;
-        }
-
-        if (!_cts.IsCancellationRequested)
-            Completed?.Invoke();
-    }
-
-    private void Fail(string message) => Console.Error.WriteLine(message);
-
-    private void KillProcess()
-    {
-        try { _active?.Kill(); } catch { /* already exited */ }
-    }
+    public void Dispose() => _clip.Dispose();
 }