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.

Services/Audio/ITraxPlayer.cs

@@ -8,6 +8,10 @@ public interface ITraxPlayer : IDisposable
     string? TrackName { get; }
     bool IsPlaying { get; }
     float LocalGain { get; set; }
+
+    /// <summary>The creator's TRAX level (1 = the bed's fixed design level). Separate from
+    /// <see cref="LocalGain"/>, which follows the desktop mute; the dock's fader moves this.</summary>
+    float UserGain { get; set; }
     void Load(string path);
     void Play();
     void Pause();