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).

ViewModels/MainViewModel.Audio.cs

@@ -56,6 +56,7 @@ public partial class MainViewModel
                 OnPropertyChanged(nameof(MicMeterFraction));
                 UpdateDuckState();
                 RaiseMasterReadouts();
+                OnPropertyChanged(nameof(TraxMeterFraction));
                 OnPropertyChanged(nameof(MicMeterFraction));
                 OnPropertyChanged(nameof(MicVolumeDb));
                 OnPropertyChanged(nameof(MicVolumeDbLabel));