feat(dock): TRAX becomes a background level, not a channel strip

Creator ruling (2026-10-30): TRAX is background music and is controlled separately from desktop and
mic — so KISS it:

- TRAX loses its own ladder and slider. It is one number plus ±5 dB buttons, disabled at the ends
  (−60 dB floor, +6 dB ceiling).
- It STARTS at −50 dB, taken from the creator's own meter readings, and moves in 5 dB clicks. If that
  proves inaudible on their setup, six clicks lands at −20 — the buttons exist for exactly that, and
  `TraxDefaultDb` is one constant.
- Layout follows: Desktop's volume now sits directly beneath its ladder, and the TRAX row (name,
  play/pause, level) sits beneath that, inside the GAME bay. The bays are MIC | GAME+TRAX | LIP-SYNC.
- The metering tap on the player stays (it is how "is the music producing sound" is answered) but has
  no ladder of its own; the desktop bed already shows the music.

Guards updated: TraxBayTests now asserts the stepper wiring and that the row has NO SegmentMeter;
GameBayTests asserts the desktop fader sits below the desktop ladder.

Controls/AudioDock.xaml.cs

@@ -39,6 +39,16 @@ public partial class AudioDock : UserControl
             vm.OpenMicPickerCommand.Execute(null);
     }
 
+    private void TraxStepDown_Click(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is ViewModels.MainViewModel vm) vm.StepTrax(-1);
+    }
+
+    private void TraxStepUp_Click(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is ViewModels.MainViewModel vm) vm.StepTrax(+1);
+    }
+
     private void SyncPreset_Click(object sender, RoutedEventArgs e)
     {
         if (DataContext is not ViewModels.MainViewModel vm) return;