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.Load.cs

@@ -113,7 +113,7 @@ public partial class LayoutStore : IDisposable
                        AlertVideoAssetId, AlertVideoPath, AlertUseDefaultVideo, AlertShowTicker, AlertVideoVolume, SortOrder,
                        AlertDisplayMethod, IsBrandLogo, BlurAmount, AlertAudioPath, AlertAudioLoop,
                         TextContent, TextFontFamily, TextFontSize, TextColor, TextBold, TextItalic,
-                        TextUnderline, TextAlignment, TextVerticalAlignment
+                        TextUnderline, TextAlignment, TextVerticalAlignment, MediaMuted
                 FROM Source ORDER BY SortOrder
                 """;
             using var reader = cmd.ExecuteReader();
@@ -183,6 +183,7 @@ public partial class LayoutStore : IDisposable
                         || !Enum.IsDefined((TextLayerVerticalAlignment)reader.GetInt32(49))
                             ? TextLayerVerticalAlignment.Top
                             : (TextLayerVerticalAlignment)reader.GetInt32(49),
+                    MediaMuted = reader.GetInt32(reader.GetOrdinal("MediaMuted")) != 0,
                 };
                 if (!sourcesByScene.TryGetValue(sceneId, out var list))
                     sourcesByScene[sceneId] = list = new List<(Source, int)>();