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).

ViewModels/MainViewModel.Media.cs

@@ -6,6 +6,7 @@ using System.Windows.Media.Imaging;
 using Microsoft.Win32;
 using ytLive.Models;
 using ytLive.Services;
+using ytLive.Services.Audio;
 
 namespace ytLive.ViewModels;
 
@@ -105,6 +106,11 @@ public partial class MainViewModel
             {
                 _mediaManager.SetLooping(path, AnyMediaReferenceLoops(path));
             }
+            else if (e.PropertyName is nameof(Source.MediaVolume) or nameof(Source.MediaMuted)
+                     && source.MediaPath is { } audioPath)
+            {
+                ApplyMediaAudioSettings(audioPath);
+            }
         }
 
         _mediaHandlers[source.Id] = OnSourceChanged;
@@ -121,7 +127,11 @@ public partial class MainViewModel
     private void EnsureMediaSession(string? path)
     {
         if (string.IsNullOrWhiteSpace(path)) return;
-        if (_mediaManager.HasSession(path)) return;
+        if (_mediaManager.HasSession(path))
+        {
+            ApplyMediaAudioSettings(path);
+            return;
+        }
         _ = AcquireMediaAsync(path);
     }
 
@@ -133,8 +143,25 @@ public partial class MainViewModel
         // case the bitmap is already published and will not be raised again for us.
         if (_mediaManager.GetPreviewBitmap(path) is { } bitmap)
             OnMediaPreviewBitmapChanged(path, bitmap);
+
+        ApplyMediaAudioSettings(path);
     }
 
+    /// <summary>The file's sound controls: one decoder is shared per path, so the level and
+    /// mute are per session — the layer that was last touched is the one that wins.</summary>
+    private void ApplyMediaAudioSettings(string path)
+    {
+        var source = Scenes.SelectMany(s => s.Elements).OfType<Source>()
+            .FirstOrDefault(s => s.IsMediaSource && s.MediaPath == path);
+        if (source == null) return;
+        _mediaManager.SetAudioSettings(path, source.MediaVolume, source.MediaMuted);
+    }
+
+    /// <summary>A clip's own soundtrack joins the desktop/game group, so the desktop fader and
+    /// the voice ducker treat it exactly like game audio (TASK 21 audio slice).</summary>
+    private void OnMediaAudioAvailable(string path, AudioSample sample)
+        => _audioMixer.EnqueueDesktopMediaAudio(sample);
+
     /// <summary>Shared sessions or not, the loop state is the path's: any live media
     /// reference asking for a loop makes the one decoder loop (recorded rule, TASK 21).</summary>
     private bool AnyMediaReferenceLoops(string path)