feat(dock): TRAX gets its own ladder — measured at the player's output

Creator: "why is there no meter for when trax is playing?"

Routing answer: TRAX rides the desktop loopback, so it already moved the GAME ladder — there was no
separate TRAX signal in the mix. But that ladder cannot answer the question a deaf creator actually
has: "is the music producing sound at all?" The play glyph cannot either — an open file with a dead
decoder still reports Playing.

So the player now measures its own output: \`MusicPlayer\` wraps the volume stage in an NAudio
\`MeteringSampleProvider\` and exposes \`Level\`; \`ITraxPlayer.Level\` keeps the fake honest. The VM maps
it through the same display scale (\`TraxMeterFraction\`), zeroed when not playing, and the TRAX bay
carries a 24-segment ladder above its fader.

This reverses my earlier "no ladder, nothing honest to show" — the honest thing arrived with a real
measurement. TraxBayTests flipped accordingly (it now asserts the ladder AND its source).

Controls/AudioDock.xaml

@@ -159,7 +159,10 @@
                             </Border>
                         </Grid>
 
-                        <DockPanel Grid.Row="1" Margin="0,6,0,0" VerticalAlignment="Bottom">
+                        <controls:SegmentMeter Grid.Row="1" Height="10" Margin="0,6,0,6" Orientation="Horizontal"
+                                               Level="{Binding TraxMeterFraction}" Segments="24"/>
+
+                        <DockPanel Grid.Row="1" Margin="0,22,0,0" VerticalAlignment="Bottom">
                             <TextBlock DockPanel.Dock="Right" Width="56" TextAlignment="Right" FontSize="10"
                                        FontFamily="{StaticResource FontFamily.Mono}" VerticalAlignment="Center"
                                        Foreground="{StaticResource Brush.OnSurfaceVariant}"