feat(dock): the TRAX bay — track, play/pause, level, duck badge

Unit 7. The bay shows the track name (left-click toggles playback, right-click opens the picker —
the label stays the control), a play/pause button, the duck badge, and a dB level.

No ladder here, deliberately: nothing measures the music bed, and a decorative ladder would break
the measured-not-painted rule. A test asserts the bay has no SegmentMeter.

The level is real: `ITraxPlayer.UserGain` is a new creator gain multiplied onto the fixed design
volume, separate from `LocalGain` (which follows the desktop mute). The VM's `TraxVolumeDb` is the
same dB scale as the other faders, so unity means the bed's design level.

Controls/AudioDock.xaml.cs

@@ -39,6 +39,21 @@ public partial class AudioDock : UserControl
             vm.OpenMicPickerCommand.Execute(null);
     }
 
+    private void TraxPlayPause_Click(object sender, RoutedEventArgs e)
+    {
+        if (DataContext is ViewModels.MainViewModel vm) vm.TraxLeftClick();
+    }
+
+    private void TraxLabel_MouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
+    {
+        if (DataContext is ViewModels.MainViewModel vm) vm.TraxLeftClick();
+    }
+
+    private void TraxLabel_MouseRightButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
+    {
+        if (DataContext is ViewModels.MainViewModel vm) vm.ChooseTrack();
+    }
+
     private void OnLoaded(object sender, RoutedEventArgs e)
     {
         _window = Window.GetWindow(this);