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();

Services/Audio/MusicPlayer.cs

@@ -41,6 +41,19 @@ public sealed class MusicPlayer : ITraxPlayer
 
     /// <summary>Local gain multiplier from the desktop/game audio bar (0 = mute,
     /// 1 = full bed). Applied live on top of the fixed <see cref="MusicVolume"/>.</summary>
+    private float _userGain = 1f;
+
+    /// <summary>Creator-set level, multiplied onto the fixed design volume.</summary>
+    public float UserGain
+    {
+        get => _userGain;
+        set
+        {
+            _userGain = Math.Clamp(value, 0f, 2f);
+            if (_output != null) _output.Volume = MusicVolume * _userGain * LocalGain;
+        }
+    }
+
     public float LocalGain
     {
         get => _localGain;
@@ -61,7 +74,7 @@ public sealed class MusicPlayer : ITraxPlayer
         Stop();
         TrackPath = path;
         _reader = new MediaFoundationReader(path);
-        _volume = new VolumeWaveProvider16(_reader) { Volume = MusicVolume * _localGain };
+        _volume = new VolumeWaveProvider16(_reader) { Volume = MusicVolume * _userGain * _localGain };
         _output = new WaveOutEvent();
         _output.PlaybackStopped += OnPlaybackStopped;
         _output.Init(_volume);