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/LayoutStore.Save.cs

@@ -94,7 +94,7 @@ public partial class LayoutStore : IDisposable
                                     AlertVideoAssetId, AlertVideoPath, AlertUseDefaultVideo, AlertShowTicker, AlertVideoVolume,
                                     AlertDisplayMethod, IsBrandLogo, BlurAmount, AlertAudioPath, AlertAudioLoop,
                                     TextContent, TextFontFamily, TextFontSize, TextColor, TextBold, TextItalic,
-                                    TextUnderline, TextAlignment, TextVerticalAlignment)
+                                    TextUnderline, TextAlignment, TextVerticalAlignment, MediaMuted)
                 VALUES ($id, $sceneId, $assetId, $type, $name, $isEnabled,
                         $x, $y, $w, $h, $opacity, $monitor,
                         $clip, $mirrored, $isBackground, $captureKey, $showDesktop, $sort,
@@ -104,7 +104,7 @@ public partial class LayoutStore : IDisposable
                         $alertVideoAssetId, $alertVideoPath, $alertUseDefaultVideo, $alertShowTicker, $alertVideoVolume,
                         $alertDisplayMethod, $isBrandLogo, $blurAmount, $alertAudioPath, $alertAudioLoop,
                         $textContent, $textFontFamily, $textFontSize, $textColor, $textBold, $textItalic,
-                        $textUnderline, $textAlignment, $textVerticalAlignment)
+                        $textUnderline, $textAlignment, $textVerticalAlignment, $mediaMuted)
                 """;
             cmd.Transaction = tx;
             var idP = cmd.Parameters.Add("$id", SqliteType.Text);
@@ -137,6 +137,7 @@ public partial class LayoutStore : IDisposable
             var mediaPathP = cmd.Parameters.Add("$mediaPath", SqliteType.Text);
             var mediaIsLoopingP = cmd.Parameters.Add("$mediaIsLooping", SqliteType.Integer);
             var mediaVolumeP = cmd.Parameters.Add("$mediaVolume", SqliteType.Real);
+            var mediaMutedP = cmd.Parameters.Add("$mediaMuted", SqliteType.Integer);
             var mediaPlaybackStateP = cmd.Parameters.Add("$mediaPlaybackState", SqliteType.Text);
             var alertVideoAssetIdP = cmd.Parameters.Add("$alertVideoAssetId", SqliteType.Text);
             var alertVideoPathP = cmd.Parameters.Add("$alertVideoPath", SqliteType.Text);
@@ -198,6 +199,7 @@ public partial class LayoutStore : IDisposable
                     mediaPathP.Value = (object?)source.MediaPath ?? DBNull.Value;
                     mediaIsLoopingP.Value = source.MediaIsLooping ? 1 : 0;
                     mediaVolumeP.Value = source.MediaVolume;
+                    mediaMutedP.Value = source.MediaMuted ? 1 : 0;
                     mediaPlaybackStateP.Value = source.MediaPlaybackState.ToString();
                     alertVideoAssetIdP.Value = (object?)source.AlertVideoAssetId ?? DBNull.Value;
                     alertVideoPathP.Value = (object?)source.AlertVideoPath ?? DBNull.Value;