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.

Services/LayoutStore.Settings.cs

@@ -44,6 +44,13 @@ public partial class LayoutStore : IDisposable
         UpsertSetting("Audio.SyncOffsetMs", Math.Clamp(ms, -500, 500).ToString());
     }
 
+    /// <summary>Mono output (creator ruling 2026-10-30): folds the mix so single-sided audio is
+    /// audible on both sides. Restored at startup — an accessibility switch that resets is a
+    /// failure, not a default.</summary>
+    public bool LoadMonoOutput() => GetSetting("Audio.MonoOutput") == "1";
+
+    public void SaveMonoOutput(bool enabled) => UpsertSetting("Audio.MonoOutput", enabled ? "1" : "0");
+
     /// <summary>The user's chosen record output folder (TASK 18), or null to use the
     /// default <c>%APPDATA%\ytLlive\recordings</c>. Restored at startup.</summary>
     public string? LoadRecordFolder()