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).

HANDOFF.md

@@ -1,20 +1,21 @@
-# HANDOFF — 2026-11-05 (TASK 21 — video file layer)
+# HANDOFF — 2026-11-05 (TASK 21 — video file layer + its audio)
 
 ## State
 
-- `main`; TASK 21 is complete: a video file added from the (+) menu decodes, composites into preview
-  and stream, loops on toggle, and stops with its last scene reference. 3·17 (text layers) completed
-  2026-11-04.
-- New test `MediaSourceAcquisitionTests` (add → one decoder starts + frames reach the layer + loop flip
-  reaches the decoder + remove → stops). App build is 0 warnings.
-- **Media is picture-only:** `MediaVideoSource` decodes with `-an`, so a video file's own soundtrack
-  is silent on stream — the loopback does not carry it because nothing plays it. Two routes recorded
-  in `task-21` item 10 (alert-style mixer feed vs TRAX-style local play); no volume slider until one exists.
+- `main`; TASK 21 is complete. A video file added from the (+) menu decodes, composites into preview and
+  stream, loops on toggle, and **plays its own soundtrack** (Mute + Level per layer, mixed into the
+  desktop/game group so the desktop fader and the ducker apply). Removing it stops the decoder.
+- Decoding is now ONE pipe: `Services/VideoClipPipe.cs` behind both `MediaVideoSource` (TASK 21) and
+  `AlertClipDecoder` (TASK 47) — "a clip is a clip". Alert behaviour is unchanged and its tests stay green.
+- Tests: `MediaSourceAcquisitionTests` (add → decoder starts, frames, loop, remove) and
+  `MediaSourceAudioTests` (chunks, level scaling, mute, sound reaching the desktop meter). App build 0 warnings.
 
 ## Next action
 
 - Next queued unit: TASK 49 (chat profanity filter). Day 3's TASK 32/33 resilience slices are still
   outstanding — 32·4 (fragmented MP4) is the smallest.
+- Agreed next feature work (designed, not started): green/blue screen detection + keying through the clip
+  pipe — suggest-only by default, an opt-in "Auto Green/Blue Screen" setting, alert clips included.
 - Do not push absent a checkpoint request.
 
 ## Landmines
@@ -23,5 +24,6 @@
   mouse input. Do not run it while the desktop is in use.
 - Integration tests constructing `MainWindow` must set a temporary `LayoutPathOverride` and clear it in `finally`.
 - Media sessions are keyed per **path**, not per layer: acquire through `EnsureMediaSession`, release
-  through `ReleaseMediaIfUnused`, never `AcquireAsync` twice for one path.
+  through `ReleaseMediaIfUnused`, never `AcquireAsync` twice for one path. Level/mute are therefore
+  per session too.
 - Bundled FFmpeg stays outside the single-file exe (`ExcludeFromSingleFile="true"`).