feat(dock): master strip + MONO output (the accessibility switch)

Unit 9, the one that matters most for this creator.

MASTER STRIP (top, nearest the preview):
- one ladder fed by the MEASURED mix — `AudioMixer` now measures the buffer the encoder receives
  (`MasterLevel`, `MasterImbalanceDb`), so the master cannot show a source instead of the output
- `-12.4 dB` readout, and ONE L/R flag that appears above a 6 dB gap instead of a second ladder
- bitrate / FPS / dropped / duration from the existing health poll

MONO OUTPUT:
- `AudioFold.ToMono` averages each stereo pair into both channels, applied after the limiter and
  the sync delay, so the stream AND the recording get what was folded
- `MonoOutput` is persisted (`Audio.MonoOutput`) and restored at startup: an accessibility switch
  that resets each session is an accessibility failure
- one click in the strip, remembered forever

Guard: AudioFoldTests (hard-left reaches the right at half amplitude, hard-right — the inaudible
case — reaches the left, centred material keeps its level, mono buffers untouched) and
MasterStripTests (the strip binds the measured mix + the switch, the store round-trips it, and the
fold is the tested helper rather than new arithmetic). 33 audio tests green.

ViewModels/MainViewModel.Audio.cs

@@ -55,6 +55,7 @@ public partial class MainViewModel
                 OnPropertyChanged(nameof(MeterBrush));
                 OnPropertyChanged(nameof(MicMeterFraction));
                 UpdateDuckState();
+                RaiseMasterReadouts();
                 OnPropertyChanged(nameof(MicMeterFraction));
                 OnPropertyChanged(nameof(MicVolumeDb));
                 OnPropertyChanged(nameof(MicVolumeDbLabel));
@@ -200,6 +201,39 @@ public partial class MainViewModel
         }
     }
 
+    // ─── Master strip ────────────────────────────────────────────────────────
+
+    private bool _monoOutput;
+
+    /// <summary>Collapse the output to mono. Persisted, because a creator who needs it needs it on
+    /// every session — an accessibility switch that resets is an accessibility failure.</summary>
+    public bool MonoOutput
+    {
+        get => _monoOutput;
+        set
+        {
+            if (!SetProperty(ref _monoOutput, value)) return;
+            _audioMixer.MonoOutput = value;
+            _layoutStore.SaveMonoOutput(value);
+        }
+    }
+
+    /// <summary>Master ladder input: the measured mix, 0…1 on the same display scale.</summary>
+    public double MasterMeterFraction => AudioLevelMeter.ToDisplay(_audioMixer.MasterLevel);
+
+    public string MasterLevelLabel => Helpers.GainDb.Label(Helpers.GainDb.ToDb(_audioMixer.MasterLevel)) + " dB";
+
+    /// <summary>True when the mix's channels differ by more than 6 dB — the one thing L/R is
+    /// genuinely good for, shown as one flag instead of two ladders.</summary>
+    public bool MasterImbalance => _audioMixer.MasterImbalanceDb > 6f;
+
+    private void RaiseMasterReadouts()
+    {
+        OnPropertyChanged(nameof(MasterMeterFraction));
+        OnPropertyChanged(nameof(MasterLevelLabel));
+        OnPropertyChanged(nameof(MasterImbalance));
+    }
+
     /// <summary>"0 ms (aligned)", "+50 ms", "−120 ms" — what the dock shows beside the slider.</summary>
     public string SyncOffsetLabel => AudioSyncOffsetMs == 0
         ? "0 ms (aligned)"