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).

ytLive.Tests/MediaVideoSourceManagerTests.cs

@@ -3,6 +3,7 @@ using System.Windows.Media.Imaging;
 using System.Windows.Threading;
 using Xunit;
 using ytLive.Services;
+using ytLive.Services.Audio;
 
 namespace ytLive.Tests;
 
@@ -22,6 +23,7 @@ public class MediaVideoSourceManagerTests
         public string Key { get; }
         public bool Looping { get; set; }
         public event Action<VideoFrame>? FrameAvailable;
+        public event Action<AudioSample>? AudioAvailable;
         public event Action? Completed;
 
         public FakeMediaSource(string key, List<string>? started = null, List<string>? stopped = null)
@@ -44,6 +46,7 @@ public class MediaVideoSourceManagerTests
         }
 
         public void Pump(VideoFrame frame) => FrameAvailable?.Invoke(frame);
+        public void EmitAudio(AudioSample sample) => AudioAvailable?.Invoke(sample);
         public void Complete() => Completed?.Invoke();
         public void Dispose() { }
     }
@@ -53,12 +56,14 @@ public class MediaVideoSourceManagerTests
         public string Key { get; }
         public bool Looping { get; set; }
         public event Action<VideoFrame>? FrameAvailable;
+        public event Action<AudioSample>? AudioAvailable;
         public event Action? Completed;
         public FailingMediaSource(string key) => Key = key;
         public Task StartAsync(CancellationToken cancellationToken = default)
             => throw new InvalidOperationException("access denied");
         public Task StopAsync() => Task.CompletedTask;
         public void Pump(VideoFrame frame) => FrameAvailable?.Invoke(frame);
+        public void EmitAudio(AudioSample sample) => AudioAvailable?.Invoke(sample);
         public void Complete() => Completed?.Invoke();
         public void Dispose() { }
     }