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.

ytLive.Tests/TraxToggleTests.cs

@@ -21,6 +21,7 @@ public sealed class TraxToggleTests
         public string? TrackName { get; private set; }
         public bool IsPlaying { get; private set; }
         public float LocalGain { get; set; } = 1f;
+        public float UserGain { get; set; } = 1f;
         public void Load(string path) => TrackName = Path.GetFileNameWithoutExtension(path);
         public void Play() => IsPlaying = Starts;
         public void Pause() => IsPlaying = false;