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);

ytLive.Tests/TraxBayTests.cs

@@ -16,20 +16,25 @@ public class TraxBayTests
     {
         var dock = SourceAssert.Text("Controls", "AudioDock.xaml");
         Assert.Contains("Text=\"{Binding TraxTrackLabel}\"", dock);
-        Assert.Contains("Value=\"{Binding TraxVolumeDb, Mode=TwoWay}\"", dock);
+        // KISS (creator 2026-10-30): background music is one number and two ±5 dB buttons, not a
+        // channel strip — and it has no ladder of its own, because the desktop bed already carries it.
+        Assert.Contains("Click=\"TraxStepDown_Click\"", dock);
+        Assert.Contains("Click=\"TraxStepUp_Click\"", dock);
+        Assert.Contains("IsEnabled=\"{Binding CanLowerTrax}\"", dock);
+        Assert.Contains("IsEnabled=\"{Binding CanRaiseTrax}\"", dock);
         Assert.Contains("Text=\"{Binding TraxVolumeDbLabel}\"", dock);
         Assert.Contains("Binding=\"{Binding IsMusicPlaying}\" Value=\"True\"", dock);
         Assert.Contains("Click=\"TraxPlayPause_Click\"", dock);
 
         // 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)
+        // The TRAX row lives under the desktop volume, inside the GAME bay.
+        var traxRow = dock[dock.IndexOf("Text=\"TRAX\"", System.StringComparison.Ordinal)
                            ..dock.IndexOf("Text=\"LIP-SYNC\"", System.StringComparison.Ordinal)];
-        Assert.Contains("TraxTrackLabel", traxBay);
-        Assert.Contains("SegmentMeter", traxBay);
-        Assert.Contains("TraxMeterFraction", traxBay);
+        Assert.Contains("TraxTrackLabel", traxRow);
+        Assert.DoesNotContain("SegmentMeter", traxRow);
 
-        foreach (var name in new[] { "TraxVolumeDb", "TraxVolumeDbLabel", "TraxTrackLabel", "IsTraxDucking", "TraxMeterFraction" })
+        foreach (var name in new[] { "TraxVolumeDb", "TraxVolumeDbLabel", "TraxTrackLabel", "CanRaiseTrax", "CanLowerTrax" })
             Assert.NotNull(typeof(MainViewModel).GetProperty(name));
 
         // The player gained a real user level, separate from the mute-follow LocalGain.
@@ -37,7 +42,9 @@ public class TraxBayTests
         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.
+        // It starts as background: −50 dB, and the ±5 dB steps are how it moves.
+        Assert.Equal(-50, MainViewModel.TraxDefaultDb);
+        Assert.Equal(5, MainViewModel.TraxStepDb);
         Assert.Equal(0, GainDb.ToDb(1f), 6);
         Assert.Contains("∞", GainDb.Label(GainDb.FloorDb));   // true silence, not "-60.0"
     }