fix(dock): meters beat on their own 20 Hz clock, and TRAX silence is diagnosed

Reported: TRAX plays audibly but no ladder registers anywhere.

Two distinct faults, one of them mine:
1. The meter readouts were refreshed only from the MIC's level callback. With a silent or absent
   mic there is no tick, so a playing TRAX never updated its ladder — the level was measured, the
   UI simply never asked for it. Meters now have a 20 Hz DispatcherTimer of their own.
2. If the player's own meter still reads 0 while playing, the samples are not reaching the device
   (or metering is not attached). That now writes ONE line to startup.log after 2s — so the next
   report says which side is broken instead of another round of guessing.

Note: TRAX rides the desktop loopback, so once the mix captures it the GAME ladder should move too;
if it does not, the loopback endpoint and the music's output device differ.

ViewModels/MainViewModel.Audio.cs

@@ -13,6 +13,49 @@ namespace ytLive.ViewModels;
 
 public partial class MainViewModel
 {
+    private readonly DispatcherTimer _meterTimer;
+    private int _traxSilentTicks;
+    private bool _traxSilenceLogged;
+
+    /// <summary>The dock's heartbeat: every meter readout is refreshed here, 20 times a second,
+    /// from whatever the audio threads last measured.</summary>
+    private void TickMeters()
+    {
+        OnPropertyChanged(nameof(MicMeterFraction));
+        OnPropertyChanged(nameof(MeterFillWidth));
+        OnPropertyChanged(nameof(MeterBrush));
+        OnPropertyChanged(nameof(GameMeterFraction));
+        OnPropertyChanged(nameof(GameMeterFillWidth));
+        OnPropertyChanged(nameof(GameMeterBrush));
+        OnPropertyChanged(nameof(TraxMeterFraction));
+        RaiseMasterReadouts();
+        UpdateDuckState();
+        CheckTraxSilence();
+    }
+
+    /// <summary>One honest diagnostic: "playing" with a flat player meter means the samples never
+    /// reached the device (or the metering was not attached). Logged once per playback, so the next
+    /// run says which side is broken instead of another round of guessing.</summary>
+    private void CheckTraxSilence()
+    {
+        if (!IsMusicPlaying)
+        {
+            _traxSilentTicks = 0;
+            _traxSilenceLogged = false;
+            return;
+        }
+        if (_musicPlayer.Level > 0f)
+        {
+            _traxSilentTicks = 0;
+            _traxSilenceLogged = false;
+            return;
+        }
+        if (++_traxSilentTicks < 40 || _traxSilenceLogged) return;   // 2s at 20 Hz
+        _traxSilenceLogged = true;
+        AppLog.Write("TRAX: playing but the player meter reads 0 — the output device is silent or " +
+                     "the metering stream is not attached (the dock's ladder has nothing to show)");
+    }
+
     private readonly DispatcherTimer _volumeFlashTimer;
     private readonly DispatcherTimer _gameVolumeFlashTimer;
     private double _audioLevel;