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.

Controls/AudioDock.xaml

@@ -214,6 +214,50 @@
                 <DockPanel VerticalAlignment="Center">
                     <TextBlock DockPanel.Dock="Left" Text="MASTER" FontSize="10" FontWeight="SemiBold"
                                Foreground="{StaticResource Brush.OnSurfaceVariant}" VerticalAlignment="Center"/>
+
+                    <!-- One ladder for the mix, plus the imbalance flag: two ladders would cost the
+                         width that the telemetry needs, and the flag is the case that matters. -->
+                    <controls:SegmentMeter DockPanel.Dock="Left" Width="120" Height="8" Margin="10,0,0,0"
+                                           Orientation="Horizontal" Segments="24"
+                                           Level="{Binding MasterMeterFraction}" VerticalAlignment="Center"/>
+                    <TextBlock DockPanel.Dock="Left" Width="62" Margin="6,0,0,0" FontSize="10"
+                               FontFamily="{StaticResource FontFamily.Mono}" VerticalAlignment="Center"
+                               Foreground="{StaticResource Brush.OnSurfaceVariant}"
+                               Text="{Binding MasterLevelLabel}"/>
+
+                    <Border DockPanel.Dock="Left" Margin="6,0,0,0" Padding="5,1" VerticalAlignment="Center"
+                            CornerRadius="{StaticResource Radius.Sm}" Background="{StaticResource Brush.SurfaceContainerHigh}"
+                            Visibility="{Binding MasterImbalance, Converter={StaticResource BoolToVis}}"
+                            ToolTip="Left and right differ by more than 6 dB — one side may be inaudible.">
+                        <TextBlock Text="L/R" FontSize="9" FontWeight="Bold" FontFamily="{StaticResource FontFamily.Mono}"
+                                   Foreground="{StaticResource Brush.Attention}"/>
+                    </Border>
+
+                    <!-- MONO: the accessibility switch, one click, remembered. -->
+                    <ToggleButton DockPanel.Dock="Left" Margin="10,0,0,0" Height="20" Padding="8,0"
+                                  Style="{StaticResource SegmentToggle}" IsChecked="{Binding MonoOutput, Mode=TwoWay}"
+                                  ToolTip="Fold the output to mono — both ears hear everything">
+                        <TextBlock Text="MONO" FontSize="10" FontWeight="Bold"
+                                   Foreground="{Binding Foreground, RelativeSource={RelativeSource AncestorType=ToggleButton}}"/>
+                    </ToggleButton>
+
+                    <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" VerticalAlignment="Center">
+                        <TextBlock FontSize="10" FontFamily="{StaticResource FontFamily.Mono}" Margin="0,0,12,0"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}">
+                            <Run Text="{Binding CurrentHealth.CurrentBitrate, StringFormat={}{0:0} kbps}"/>
+                        </TextBlock>
+                        <TextBlock FontSize="10" FontFamily="{StaticResource FontFamily.Mono}" Margin="0,0,12,0"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}">
+                            <Run Text="{Binding CurrentHealth.FPS, StringFormat={}{0:0} fps}"/>
+                        </TextBlock>
+                        <TextBlock FontSize="10" FontFamily="{StaticResource FontFamily.Mono}" Margin="0,0,12,0"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}">
+                            <Run Text="drop "/><Run Text="{Binding CurrentHealth.DroppedFrames}"/>
+                        </TextBlock>
+                        <TextBlock FontSize="10" FontFamily="{StaticResource FontFamily.Mono}"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}"
+                                   Text="{Binding CurrentHealth.StreamDuration, StringFormat={}{0:hh\:mm\:ss}}"/>
+                    </StackPanel>
                 </DockPanel>
             </Border>
         </Grid>