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.cs

@@ -269,6 +269,7 @@ public partial class MainViewModel : ViewModelBase
         // (yellow) instead of silently falling back to the default.
         MicSourceName = _layoutStore.LoadMicSourceName();
         AudioSyncOffsetMs = _layoutStore.LoadAudioSyncOffsetMs();
+        MonoOutput = _layoutStore.LoadMonoOutput();   // applied to the mixer by the setter
         try
         {
             var s = _layoutStore.GetSetting("ZoomShape");