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).

ytLive.Tests/TraxBayTests.cs

@@ -21,17 +21,20 @@ public class TraxBayTests
         Assert.Contains("Binding=\"{Binding IsMusicPlaying}\" Value=\"True\"", dock);
         Assert.Contains("Click=\"TraxPlayPause_Click\"", dock);
 
-        // No ladder in the TRAX bay: nothing measures the bed, so there is nothing honest to show.
+        // The bay DOES carry a ladder now — the player measures its own output, so "is music making
+        // sound" is answerable without hearing it (a play glyph cannot answer that).
         var traxBay = dock[dock.IndexOf("Text=\"TRAX\"", System.StringComparison.Ordinal)
                            ..dock.IndexOf("Text=\"LIP-SYNC\"", System.StringComparison.Ordinal)];
         Assert.Contains("TraxTrackLabel", traxBay);
-        Assert.DoesNotContain("SegmentMeter", traxBay);
+        Assert.Contains("SegmentMeter", traxBay);
+        Assert.Contains("TraxMeterFraction", traxBay);
 
-        foreach (var name in new[] { "TraxVolumeDb", "TraxVolumeDbLabel", "TraxTrackLabel", "IsTraxDucking" })
+        foreach (var name in new[] { "TraxVolumeDb", "TraxVolumeDbLabel", "TraxTrackLabel", "IsTraxDucking", "TraxMeterFraction" })
             Assert.NotNull(typeof(MainViewModel).GetProperty(name));
 
         // The player gained a real user level, separate from the mute-follow LocalGain.
         Assert.NotNull(typeof(ITraxPlayer).GetProperty("UserGain"));
+        Assert.NotNull(typeof(ITraxPlayer).GetProperty("Level"));
         Assert.NotNull(typeof(MusicPlayer).GetProperty("UserGain"));
 
         // Unity means the bed's design level, and the fader's scale is the same one as the others.