feat(dock): TRAX gets its own ladder — measured at the player's output

Creator: "why is there no meter for when trax is playing?"

Routing answer: TRAX rides the desktop loopback, so it already moved the GAME ladder — there was no
separate TRAX signal in the mix. But that ladder cannot answer the question a deaf creator actually
has: "is the music producing sound at all?" The play glyph cannot either — an open file with a dead
decoder still reports Playing.

So the player now measures its own output: \`MusicPlayer\` wraps the volume stage in an NAudio
\`MeteringSampleProvider\` and exposes \`Level\`; \`ITraxPlayer.Level\` keeps the fake honest. The VM maps
it through the same display scale (\`TraxMeterFraction\`), zeroed when not playing, and the TRAX bay
carries a 24-segment ladder above its fader.

This reverses my earlier "no ladder, nothing honest to show" — the honest thing arrived with a real
measurement. TraxBayTests flipped accordingly (it now asserts the ladder AND its source).

Services/Audio/MusicPlayer.cs

@@ -1,5 +1,7 @@
 using System.IO;
+using System.Linq;
 using NAudio.Wave;
+using NAudio.Wave.SampleProviders;
 
 namespace ytLive.Services.Audio;
 
@@ -39,6 +41,11 @@ public sealed class MusicPlayer : ITraxPlayer
 
     public bool IsPlaying => _output?.PlaybackState == PlaybackState.Playing;
 
+    /// <summary>Measured peak of what the player is actually sending to the device.</summary>
+    public float Level { get; private set; }
+
+    private MeteringSampleProvider? _metering;
+
     /// <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;
@@ -75,9 +82,12 @@ public sealed class MusicPlayer : ITraxPlayer
         TrackPath = path;
         _reader = new MediaFoundationReader(path);
         _volume = new VolumeWaveProvider16(_reader) { Volume = MusicVolume * _userGain * _localGain };
+        _metering = new MeteringSampleProvider(_volume.ToSampleProvider());
+        _metering.StreamVolume += (_, e) =>
+            Level = e.MaxSampleValues.Length == 0 ? 0f : e.MaxSampleValues.Max();
         _output = new WaveOutEvent();
         _output.PlaybackStopped += OnPlaybackStopped;
-        _output.Init(_volume);
+        _output.Init(_metering);
     }
 
     public void Play()