feat(dock): the TRAX bay — track, play/pause, level, duck badge

Unit 7. The bay shows the track name (left-click toggles playback, right-click opens the picker —
the label stays the control), a play/pause button, the duck badge, and a dB level.

No ladder here, deliberately: nothing measures the music bed, and a decorative ladder would break
the measured-not-painted rule. A test asserts the bay has no SegmentMeter.

The level is real: `ITraxPlayer.UserGain` is a new creator gain multiplied onto the fixed design
volume, separate from `LocalGain` (which follows the desktop mute). The VM's `TraxVolumeDb` is the
same dB scale as the other faders, so unity means the bed's design level.

ytLive.Tests/TraxBayTests.cs

@@ -0,0 +1,41 @@
+using Xunit;
+using ytLive.Helpers;
+using ytLive.Services.Audio;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>The TRAX bay shows the track, plays/pauses it, and carries a level — but no meter:
+/// nothing measures the bed, and a decorative ladder would be a lie. Its fader moves a gain the
+/// player actually applies, separate from the desktop-mute follow.
+/// </summary>
+public class TraxBayTests
+{
+    [Fact]
+    public void The_trax_bay_binds_the_track_level_and_play_state_without_inventing_a_meter()
+    {
+        var dock = SourceAssert.Text("Controls", "AudioDock.xaml");
+        Assert.Contains("Text=\"{Binding TraxTrackLabel}\"", dock);
+        Assert.Contains("Value=\"{Binding TraxVolumeDb, Mode=TwoWay}\"", dock);
+        Assert.Contains("Text=\"{Binding TraxVolumeDbLabel}\"", dock);
+        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.
+        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);
+
+        foreach (var name in new[] { "TraxVolumeDb", "TraxVolumeDbLabel", "TraxTrackLabel", "IsTraxDucking" })
+            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(MusicPlayer).GetProperty("UserGain"));
+
+        // Unity means the bed's design level, and the fader's scale is the same one as the others.
+        Assert.Equal(0, GainDb.ToDb(1f), 6);
+        Assert.Contains("∞", GainDb.Label(GainDb.FloorDb));   // true silence, not "-60.0"
+    }
+}