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).

Controls/LeftPanel.xaml

@@ -746,7 +746,20 @@
                                 <CheckBox Style="{StaticResource PropertyPillCheck}" Margin="10,4,0,4"
                                           Content="Loop" IsChecked="{Binding SelectedElement.MediaIsLooping, Mode=TwoWay}"
                                           ToolTip="Restart from the beginning at the end of the file — for starting-soon and BRB loops"/>
-                                <TextBlock Text="Picture only — a video's own sound is not played yet."
+                                <DockPanel Margin="10,0,0,4">
+                                    <CheckBox DockPanel.Dock="Left" Style="{StaticResource PropertyPillCheck}"
+                                              Content="Mute" IsChecked="{Binding SelectedElement.MediaMuted, Mode=TwoWay}"
+                                              ToolTip="Silence this layer's sound. The picture keeps playing."/>
+                                </DockPanel>
+                                <DockPanel Margin="10,0,0,4">
+                                    <TextBlock DockPanel.Dock="Left" Text="Level" Style="{StaticResource PropertyFieldLabel}" Width="40"/>
+                                    <TextBlock DockPanel.Dock="Right" Text="{Binding SelectedElement.MediaVolume, StringFormat={}{0:P0}}"
+                                               Style="{StaticResource PropertyFieldLabel}" Width="34" TextAlignment="Right"/>
+                                    <Slider Minimum="0" Maximum="1" Width="120"
+                                            Value="{Binding SelectedElement.MediaVolume, Mode=TwoWay}"
+                                            VerticalAlignment="Center"/>
+                                </DockPanel>
+                                <TextBlock Text="Sound plays through the desktop audio controls, like game audio."
                                            Style="{StaticResource PropertyHint}"/>
                             </StackPanel>