feat(dock): TRAX becomes a background level, not a channel strip

Creator ruling (2026-10-30): TRAX is background music and is controlled separately from desktop and
mic — so KISS it:

- TRAX loses its own ladder and slider. It is one number plus ±5 dB buttons, disabled at the ends
  (−60 dB floor, +6 dB ceiling).
- It STARTS at −50 dB, taken from the creator's own meter readings, and moves in 5 dB clicks. If that
  proves inaudible on their setup, six clicks lands at −20 — the buttons exist for exactly that, and
  `TraxDefaultDb` is one constant.
- Layout follows: Desktop's volume now sits directly beneath its ladder, and the TRAX row (name,
  play/pause, level) sits beneath that, inside the GAME bay. The bays are MIC | GAME+TRAX | LIP-SYNC.
- The metering tap on the player stays (it is how "is the music producing sound" is answered) but has
  no ladder of its own; the desktop bed already shows the music.

Guards updated: TraxBayTests now asserts the stepper wiring and that the row has NO SegmentMeter;
GameBayTests asserts the desktop fader sits below the desktop ladder.

ytLive.Tests/GameBayTests.cs

@@ -15,6 +15,10 @@ public class GameBayTests
     {
         var dock = SourceAssert.Text("Controls", "AudioDock.xaml");
         Assert.Contains("Value=\"{Binding GameVolumeDb, Mode=TwoWay}\"", dock);
+        // Desktop volume sits directly beneath its meter (creator's layout ruling).
+        var desktopMeter = dock.IndexOf("Level=\"{Binding GameMeterFraction}\"", System.StringComparison.Ordinal);
+        var desktopFader = dock.IndexOf("Value=\"{Binding GameVolumeDb, Mode=TwoWay}\"", System.StringComparison.Ordinal);
+        Assert.True(desktopFader > desktopMeter, "the desktop fader belongs under the desktop ladder");
         Assert.Contains("Level=\"{Binding GameMeterFraction}\"", dock);
         Assert.Contains("Text=\"{Binding GameDuckLabel}\"", dock);
         Assert.Contains("Visibility=\"{Binding IsGameDucking, Converter={StaticResource BoolToVis}}\"", dock);