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/MediaVideoSourceManager.cs

@@ -6,6 +6,7 @@ using System.Windows.Media;
 using System.Windows.Media.Imaging;
 using System.Windows.Threading;
 using ytLive.Helpers;
+using ytLive.Services.Audio;
 
 namespace ytLive.Services;
 
@@ -23,8 +24,11 @@ public sealed class MediaVideoSourceManager : IDisposable
         public string Key { get; }
         public IMediaFrameSource Source { get; }
         public Action<VideoFrame>? FrameHandler;
+        public Action<AudioSample>? AudioHandler;
         public int RefCount;
         public bool Started;
+        public bool AudioMuted;
+        public double AudioVolume = 1.0;
         public WriteableBitmap? PreviewBitmap;
         public VideoFrame? LatestFrame;
         public bool FramePending;
@@ -49,6 +53,10 @@ public sealed class MediaVideoSourceManager : IDisposable
     /// <summary>Raised when a media decoder cannot be created (bad path).</summary>
     public event Action<string, string>? MediaFailed;
 
+    /// <summary>Raised (worker thread) per audio chunk of a file's own soundtrack, already
+    /// scaled by the session's level and dropped when muted.</summary>
+    public event Action<string, AudioSample>? AudioAvailable;
+
     public MediaVideoSourceManager(
         Func<string, IMediaFrameSource?> sourceFactory,
         Dispatcher? uiDispatcher = null,
@@ -95,6 +103,8 @@ public sealed class MediaVideoSourceManager : IDisposable
                 session = new MediaSession(key, source);
                 session.FrameHandler = frame => OnFrameAvailable(session, frame);
                 session.Source.FrameAvailable += session.FrameHandler;
+                session.AudioHandler = sample => OnAudioAvailable(session, sample);
+                session.Source.AudioAvailable += session.AudioHandler;
                 // Loop intent lives on the Source (per scene), so the caller supplies the
                 // per-path answer ("any live reference loops"). Applied before Start so a
                 // zero-length or instantly-EOF file cannot slip past a later assignment.
@@ -117,6 +127,7 @@ public sealed class MediaVideoSourceManager : IDisposable
             lock (_gate)
                 _sessions.Remove(key);
             session.Source.FrameAvailable -= session.FrameHandler;
+            if (session.AudioHandler != null) session.Source.AudioAvailable -= session.AudioHandler;
             AppLog.Write($"MediaSource: failed to start decoder '{key}': {ex.Message}");
             MediaFailed?.Invoke(key, ex.Message);
             await SafeStopAsync(session.Source);
@@ -137,6 +148,7 @@ public sealed class MediaVideoSourceManager : IDisposable
 
         if (toStop == null) return;
         toStop.Source.FrameAvailable -= toStop.FrameHandler;
+        if (toStop.AudioHandler != null) toStop.Source.AudioAvailable -= toStop.AudioHandler;
         await SafeStopAsync(toStop.Source);
         toStop.PreviewBitmap = null;
     }
@@ -155,6 +167,7 @@ public sealed class MediaVideoSourceManager : IDisposable
 
         if (toStop == null) return;
         toStop.Source.FrameAvailable -= toStop.FrameHandler;
+        if (toStop.AudioHandler != null) toStop.Source.AudioAvailable -= toStop.AudioHandler;
         await SafeStopAsync(toStop.Source);
         toStop.PreviewBitmap = null;
     }
@@ -193,6 +206,46 @@ public sealed class MediaVideoSourceManager : IDisposable
         }
     }
 
+    /// <summary>The layer's sound controls for this file. One decoder is shared per path,
+    /// so the level/mute are per SESSION: whichever layer the creator last touched wins —
+    /// the alternative (one decoder per layer) would play the same file twice.</summary>
+    public void SetAudioSettings(string key, double volume, bool muted)
+    {
+        lock (_gate)
+        {
+            if (!_sessions.TryGetValue(key, out var session)) return;
+            session.AudioVolume = Math.Clamp(volume, 0, 1);
+            session.AudioMuted = muted;
+        }
+    }
+
+    private void OnAudioAvailable(MediaSession session, AudioSample sample)
+    {
+        if (!TryGetActiveSession(session)) return;
+
+        double volume;
+        bool muted;
+        lock (_gate)
+        {
+            volume = session.AudioVolume;
+            muted = session.AudioMuted;
+        }
+        if (muted || volume <= 0) return;
+
+        if (volume < 0.999)
+        {
+            // Copy before scaling: the source owns its buffer and may recycle it.
+            var scaled = new float[sample.Samples.Length];
+            var gain = (float)volume;
+            for (var i = 0; i < scaled.Length; i++)
+                scaled[i] = sample.Samples[i] * gain;
+            AudioAvailable?.Invoke(session.Key, new AudioSample(scaled, sample.SampleRate, sample.Channels));
+            return;
+        }
+
+        AudioAvailable?.Invoke(session.Key, sample);
+    }
+
     /// <summary>The shared preview bitmap for a file (so a config created after the
     /// first frame can adopt the running bitmap), or null.</summary>
     public WriteableBitmap? GetPreviewBitmap(string key)
@@ -213,6 +266,7 @@ public sealed class MediaVideoSourceManager : IDisposable
         foreach (var session in sessions)
         {
             session.Source.FrameAvailable -= session.FrameHandler;
+            if (session.AudioHandler != null) session.Source.AudioAvailable -= session.AudioHandler;
             _ = SafeStopAsync(session.Source);
         }
     }