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).

docs/ai/output.md

@@ -250,6 +250,13 @@ devices, no timers).
   `StopLive()` (closes the pipe → ffmpeg audio EOF) **BEFORE** stopping the frame pump (video EOF) —
   the reverse order stalls on pipe backpressure. `FfmpegEncoder` itself is untouched; the VM owns the
   pipe lifecycle.
+  **Two clip-audio rings join that mix (2026-11-05):** `EnqueueAlertAudio` (TASK 47) is pre-scaled by the
+  alert layer and mixed at **unity, never ducked** — an alert must be heard over the mic.
+  `EnqueueDesktopMediaAudio` (TASK 21's audio slice) is the opposite on purpose: a video layer's
+  soundtrack joins the **desktop/game ring**, so the desktop fader and the auto-ducker apply exactly as
+  they do for game audio and TRAX. Both are fed by the shared `Services/VideoClipPipe.cs`; the layer's
+  own Mute/Level are applied upstream by `MediaVideoSourceManager` (level/mute are per **session**, since
+  one decoder is shared per file path).
 - **Audio sync offset (TASK 22):** after the limiter, `FillAndMix` routes the whole interleaved stereo
   mix through a **`AudioSyncDelay`** (`Services/Audio/AudioSyncDelay.cs`) — a pure delay line whose
   offset comes from a `Func<int> syncOffsetMs` seam (the VM's global `AudioSyncOffsetMs`, **−500..+500