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.

ytLive.Tests/MasterStripTests.cs

@@ -0,0 +1,41 @@
+using Xunit;
+using ytLive.Helpers;
+using ytLive.Services.Audio;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>The master strip is where the accessibility switch lives, so its wiring matters more
+/// than its looks: MONO must reach the mixer, be remembered across sessions, and the strip's ladder
+/// must show the MEASURED mix — with one L/R flag instead of two ladders.
+/// </summary>
+public class MasterStripTests
+{
+    [Fact]
+    public void The_master_strip_shows_the_measured_mix_and_the_mono_switch_is_persisted()
+    {
+        var dock = SourceAssert.Text("Controls", "AudioDock.xaml");
+        Assert.Contains("IsChecked=\"{Binding MonoOutput, Mode=TwoWay}\"", dock);
+        Assert.Contains("Level=\"{Binding MasterMeterFraction}\"", dock);
+        Assert.Contains("Text=\"{Binding MasterLevelLabel}\"", dock);
+        Assert.Contains("Visibility=\"{Binding MasterImbalance, Converter={StaticResource BoolToVis}}\"", dock);
+        // One ladder on the master, not a pair.
+        Assert.Equal(1, System.Text.RegularExpressions.Regex.Matches(
+            dock[dock.IndexOf("MASTER", System.StringComparison.Ordinal)..], "SegmentMeter").Count);
+
+        // The switch reaches the mixer and the store.
+        Assert.NotNull(typeof(AudioMixer).GetProperty("MonoOutput"));
+        Assert.NotNull(typeof(MainViewModel).GetProperty("MonoOutput"));
+        var settings = SourceAssert.Text("Services", "LayoutStore.Settings.cs");
+        Assert.Contains("Audio.MonoOutput", settings);
+        Assert.Contains("LoadMonoOutput", settings);
+
+        // The mixer measures the finished mix, so the ladder cannot show a source instead of the output.
+        Assert.NotNull(typeof(AudioMixer).GetProperty("MasterLevel"));
+        Assert.NotNull(typeof(AudioMixer).GetProperty("MasterImbalanceDb"));
+
+        // And the fold is the tested helper, not new arithmetic in the mixer.
+        Assert.Contains("AudioFold.ToMono", SourceAssert.Text("Services", "Audio", "AudioMixer.cs"));
+        Assert.Equal(0.25f, GainDb.MonoFold(0.5f, 0f));
+    }
+}