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/ITraxPlayer.cs

@@ -7,6 +7,10 @@ public interface ITraxPlayer : IDisposable
     event Action<Exception?>? PlaybackEnded;
     string? TrackName { get; }
     bool IsPlaying { get; }
+
+    /// <summary>The player's OWN measured output level (0…1). A play glyph can lie — an open file
+    /// with a dead decoder still says "playing" — so the dock's TRAX ladder shows sound, not state.</summary>
+    float Level { get; }
     float LocalGain { get; set; }
 
     /// <summary>The creator's TRAX level (1 = the bed's fixed design level). Separate from