fix(dock): the master ladder moves when no session is running

Reported: no activity in the master meter.

Cause: `MasterLevel` measures the buffer the ENCODER receives, and that buffer only exists while the
mix is being pulled — streaming or recording. Idle, nothing fills it, so the master read zero while
the bays below it moved. A master that is dead between sessions is a gap in the design.

Now: while a session runs the master shows the measured mix (unchanged); idle it shows an estimate
computed from the same measured sources and the same gains the mix applies
(`MeterScale.MonitorFraction`, pure + tested). Two ladders can therefore never tell contradictory
stories about whether audio exists.

Guard: MeterScaleTests pins the estimate — silence reads zero, a quiet source at unity is visible,
two sources add, and gain is applied in the LINEAR domain like the mix (scaling the display fraction
would be a different, wrong curve).

ViewModels/MainViewModel.Audio.cs

@@ -262,8 +262,14 @@ public partial class MainViewModel
         }
     }
 
-    /// <summary>Master ladder input: the measured mix, 0…1 on the same display scale.</summary>
-    public double MasterMeterFraction => AudioLevelMeter.ToDisplay(_audioMixer.MasterLevel);
+    /// <summary>Master ladder input: the MEASURED mix while a session runs (that is the buffer the
+    /// encoder receives), and an estimate from the measured sources when idle — otherwise the master
+    /// sits at zero between sessions while every bay below it moves.</summary>
+    public double MasterMeterFraction =>
+        IsLive || IsRecording
+            ? AudioLevelMeter.ToDisplay(_audioMixer.MasterLevel)
+            : Helpers.MeterScale.MonitorFraction((float)AudioLevel, MicVolume,
+                                                 (float)GameAudioLevel, GameAudioVolume);
 
     public string MasterLevelLabel => Helpers.GainDb.Label(Helpers.GainDb.ToDb(_audioMixer.MasterLevel)) + " dB";