feat(topbar): the segmented mode switcher, to the mock

Implements the creator's mock (2026-11-12) for the world segment: pill track (#1E222B, radius 12,
padding 4) holding two equal halves; the active half sits on a lighter pill (#2E3440, radius 8) with
#E2E8F0 bold text and a coral (#F26D6D) icon; the inactive half is #8892A4 on a one-alpha black face so
the whole area hit-tests; hover tints #252A34 on the inactive half only, so the active one never looks
clickable-but-dim. Labels are "Build scenes" / "Record / go live".

Two deliberate deviations from the pasted recipe, both to honour this repo:
- State is the existing BarWorld binding plus a command, not a code-behind SetActiveMode — one source of
  truth, and the change still raises the same property-changed notification a mode event would.
- Styles live in Themes/Controls.xaml (SegmentHalf, SegmentIcon) with colours as tokens in Tokens.xaml,
  rather than inline properties on the elements: AGENTS requires shared styles, and one DataTrigger on
  each element's own Tag serves both halves.

DesignTokensTests caught the one raw literal (#01000000) and it is now Brush.SegmentHitArea, which is the
rule working as intended.

680 pass, 0 warnings. Scope justification: Assets/starting-background.jpg is outside this unit — the
creator's replacement art, still awaiting their visual check.

Controls/TopBar.xaml

@@ -66,43 +66,40 @@
                  question "which world am I in?" and "can I edit?" cannot disagree. Clicking the
                  highlighted half re-enters it, which is how a creator gets back to the layers
                  during a session. -->
-            <Border Background="{StaticResource Brush.SurfaceContainerLowest}" CornerRadius="4"
-                    Padding="2" Height="32" VerticalAlignment="Center" Margin="0,0,12,0"
+            <!-- The segmented mode switcher (creator mock 2026-11-12): pill track, two equal halves,
+                 active half on a lighter pill with off-white bold text and a coral icon. Hover tints
+                 only the inactive half. -->
+            <Border Background="{StaticResource Brush.SegmentTrack}" CornerRadius="12" Padding="4"
+                    VerticalAlignment="Center" Margin="0,0,12,0"
                     Visibility="{Binding IsBuildWorld, Converter={StaticResource BoolToVis}}">
-                <StackPanel Orientation="Horizontal">
-                    <!-- Build Scenes carries a LAYOUT glyph while it is the active world and a PADLOCK
-                         once Record / Go Live is: the door is still there, it is just behind the
-                         session. Clicking it swaps the glyph back and takes the creator into the
-                         layers (creator ruling 2026-11-12). -->
-                    <Button Style="{StaticResource WorldSegmentButton}" Tag="{Binding IsBuildWorld}"
-                            Command="{Binding EnterBuildWorldCommand}" Padding="12,0"
+                <Grid>
+                    <Grid.ColumnDefinitions>
+                        <ColumnDefinition Width="*"/>
+                        <ColumnDefinition Width="*"/>
+                    </Grid.ColumnDefinitions>
+                    <Button Grid.Column="0" Style="{StaticResource SegmentHalf}"
+                            Tag="{Binding IsBuildWorld}" Command="{Binding EnterBuildWorldCommand}"
                             ToolTip="Edit layers, scenes and webcam placement">
                         <StackPanel Orientation="Horizontal">
-                            <Path Width="14" Height="14" Stretch="Uniform" VerticalAlignment="Center"
-                                  Fill="{Binding Foreground, RelativeSource={RelativeSource AncestorType=Button}}"
-                                  Visibility="{Binding IsBuildWorld, Converter={StaticResource BoolToVis}}"
+                            <Path Style="{StaticResource SegmentIcon}" Tag="{Binding IsBuildWorld}"
                                   Data="M3,4h8v6H3V4zM13,4h8v6h-8V4zM3,12h8v8H3V12zM13,12h8v8h-8V12z"/>
-
-                            <TextBlock Text="Build Scenes" FontSize="12" FontWeight="Bold" Margin="6,0,0,0"
+                            <TextBlock Text="Build scenes" FontSize="12" Margin="6,0,0,0"
                                        VerticalAlignment="Center"
                                        Foreground="{Binding Foreground, RelativeSource={RelativeSource AncestorType=Button}}"/>
                         </StackPanel>
                     </Button>
-                    <!-- Record / Go Live: a record dot in a ring, the same "armed" language the REC sign
-                         uses, so the two halves are read apart at a glance. -->
-                    <Button Style="{StaticResource WorldSegmentButton}" Tag="{Binding IsOutputWorld}"
-                            Command="{Binding EnterOutputWorldCommand}" Padding="12,0"
+                    <Button Grid.Column="1" Style="{StaticResource SegmentHalf}"
+                            Tag="{Binding IsOutputWorld}" Command="{Binding EnterOutputWorldCommand}"
                             ToolTip="Choose the output and start the session">
                         <StackPanel Orientation="Horizontal">
-                            <Path Width="14" Height="14" Stretch="Uniform" VerticalAlignment="Center"
-                                  Fill="{Binding Foreground, RelativeSource={RelativeSource AncestorType=Button}}"
+                            <Path Style="{StaticResource SegmentIcon}" Tag="{Binding IsOutputWorld}"
                                   Data="M12,2a10,10 0 1,0 0,20a10,10 0 1,0 0,-20zM12,4a8,8 0 1,1 0,16a8,8 0 1,1 0,-16zM12,7.5a4.5,4.5 0 1,1 0,9a4.5,4.5 0 1,1 0,-9z"/>
-                            <TextBlock Text="Record / Go Live" FontSize="12" FontWeight="Bold" Margin="6,0,0,0"
+                            <TextBlock Text="Record / go live" FontSize="12" Margin="6,0,0,0"
                                        VerticalAlignment="Center"
                                        Foreground="{Binding Foreground, RelativeSource={RelativeSource AncestorType=Button}}"/>
                         </StackPanel>
                     </Button>
-                </StackPanel>
+                </Grid>
             </Border>
             <!-- IDLE: the mode switch + the action(s) of the armed world. The three
                  controls share one height (build 77 feedback) so they read as one

Themes/Controls.xaml

@@ -1002,6 +1002,56 @@
     <!-- REC / ON-AIR segment pills: an armed session control reads red, because red is what
          "live or recording" means (DESIGN.md). Kept separate from SegmentToggle so the Rect|Circle
          choice in the properties panel stays the interactive blue. -->
+    <!-- The segmented mode switcher's icon: muted when its segment is inactive, coral when active.
+         Tag carries the segment's own flag so one style serves both halves (creator mock 2026-11-12). -->
+    <Style x:Key="SegmentIcon" TargetType="Path">
+        <Setter Property="Fill" Value="{StaticResource Brush.SegmentTextInactive}"/>
+        <Setter Property="Width" Value="15"/>
+        <Setter Property="Height" Value="15"/>
+        <Setter Property="Stretch" Value="Uniform"/>
+        <Setter Property="VerticalAlignment" Value="Center"/>
+        <Style.Triggers>
+            <DataTrigger Binding="{Binding Tag, RelativeSource={RelativeSource Self}}" Value="True">
+                <Setter Property="Fill" Value="{StaticResource Brush.SegmentIconActive}"/>
+            </DataTrigger>
+        </Style.Triggers>
+    </Style>
+
+    <!-- One segment of the switcher: pill container is the host Border, this is a half of it. -->
+    <Style x:Key="SegmentHalf" TargetType="Button">
+        <Setter Property="Background" Value="{StaticResource Brush.SegmentHitArea}"/>
+        <Setter Property="Foreground" Value="{StaticResource Brush.SegmentTextInactive}"/>
+        <Setter Property="BorderThickness" Value="0"/>
+        <Setter Property="Cursor" Value="Hand"/>
+        <Setter Property="HorizontalContentAlignment" Value="Center"/>
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Button">
+                    <Border x:Name="Face" Background="{TemplateBinding Background}"
+                            CornerRadius="8" Padding="14,6">
+                        <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
+                    </Border>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+        <Style.Triggers>
+            <!-- Active: lighter inner pill, off-white bold text. -->
+            <DataTrigger Binding="{Binding Tag, RelativeSource={RelativeSource Self}}" Value="True">
+                <Setter Property="Background" Value="{StaticResource Brush.SegmentActive}"/>
+                <Setter Property="Foreground" Value="{StaticResource Brush.SegmentTextActive}"/>
+                <Setter Property="FontWeight" Value="Bold"/>
+            </DataTrigger>
+            <!-- Hover tints ONLY the inactive half, so the active one never looks clickable-but-dim. -->
+            <MultiDataTrigger>
+                <MultiDataTrigger.Conditions>
+                    <Condition Binding="{Binding IsMouseOver, RelativeSource={RelativeSource Self}}" Value="True"/>
+                    <Condition Binding="{Binding Tag, RelativeSource={RelativeSource Self}}" Value="False"/>
+                </MultiDataTrigger.Conditions>
+                <Setter Property="Background" Value="{StaticResource Brush.SegmentHover}"/>
+            </MultiDataTrigger>
+        </Style.Triggers>
+    </Style>
+
     <!-- The top bar's world segment (creator ruling 2026-11-12): [ Build Scenes ][ Record / Go Live ].
          A Button, not a ToggleButton — the highlight follows the view model, so a click must never be
          what sets it. Each button passes its own world flag in Tag and one DataTrigger paints it. -->

Themes/Tokens.xaml

@@ -35,6 +35,16 @@
     <SolidColorBrush x:Key="Brush.Tertiary" Color="#38bdf8"/>
     <SolidColorBrush x:Key="Brush.OnTertiary" Color="#0b1326"/>
     <SolidColorBrush x:Key="Brush.OnAccent" Color="#ffffff"/>
+    <!-- Segmented mode switcher (creator mock, 2026-11-12) -->
+    <SolidColorBrush x:Key="Brush.SegmentTrack" Color="#1E222B"/>
+    <SolidColorBrush x:Key="Brush.SegmentActive" Color="#2E3440"/>
+    <SolidColorBrush x:Key="Brush.SegmentHover" Color="#252A34"/>
+    <SolidColorBrush x:Key="Brush.SegmentTextActive" Color="#E2E8F0"/>
+    <SolidColorBrush x:Key="Brush.SegmentTextInactive" Color="#8892A4"/>
+    <SolidColorBrush x:Key="Brush.SegmentIconActive" Color="#F26D6D"/>
+    <!-- One-alpha black: transparent to the eye, opaque to hit-testing, so the whole segment is clickable. -->
+    <SolidColorBrush x:Key="Brush.SegmentHitArea" Color="#01000000"/>
+
     <SolidColorBrush x:Key="Brush.Primary" Color="#ff5451"/>
     <SolidColorBrush x:Key="Brush.PrimaryContainer" Color="#b91a24"/>
     <SolidColorBrush x:Key="Brush.Secondary" Color="#ffb95f"/>

ytLive.Tests/UiShellTests.cs

@@ -29,8 +29,8 @@ public class UiShellTests
 
         // 2. The world segment replaced the EDIT toggle (creator ruling 2026-11-12): both halves are
         //    present, each lit by its own world flag through the shared style.
-        Assert.Contains("Text=\"Build Scenes\"", topBar);
-        Assert.Contains("Text=\"Record / Go Live\"", topBar);
+        Assert.Contains("Text=\"Build scenes\"", topBar);
+        Assert.Contains("Text=\"Record / go live\"", topBar);
         Assert.Contains("Tag=\"{Binding IsBuildWorld}\"", topBar);
         Assert.Contains("Tag=\"{Binding IsOutputWorld}\"", topBar);
         Assert.DoesNotContain("x:Name=\"EditBody\"", topBar);
@@ -38,6 +38,10 @@ public class UiShellTests
         // 2b. The segment is Build-world only: choosing Record / Go Live REPLACES it with the output
         //     line (creator ruling 2026-11-12), and a chevron is the way back.
         Assert.Contains("Data=\"M3,4h8v6H3V4zM13,4h8v6h-8V4zM3,12h8v8H3V12zM13,12h8v8h-8V12z\"", topBar);
+        // Segmented switcher (creator mock 2026-11-12): pill track, two equal halves, coral active icon.
+        Assert.Contains("Brush.SegmentTrack", topBar);
+        Assert.Contains("Style=\"{StaticResource SegmentHalf}\"", topBar);
+        Assert.Contains("Style=\"{StaticResource SegmentIcon}\"", topBar);
         Assert.Contains("Visibility=\"{Binding IsBuildWorld, Converter={StaticResource BoolToVis}}\"", topBar);
         Assert.Contains("Text=\"Output:\"", topBar);
         Assert.Contains("Text=\"press <esc> to return\"", topBar);
@@ -116,7 +120,7 @@ public class UiShellTests
 
         // The world segment leads the centre cluster; the brand column carries no edit control.
         var centre = topBar.IndexOf("<StackPanel Grid.Column=\"1\"", System.StringComparison.Ordinal);
-        var segment = topBar.IndexOf("Text=\"Build Scenes\"", System.StringComparison.Ordinal);
+        var segment = topBar.IndexOf("Text=\"Build scenes\"", System.StringComparison.Ordinal);
         Assert.True(centre > 0 && segment > centre,
             "the [ Build Scenes ][ Record / Go Live ] segment must lead the centre cluster");
         Assert.DoesNotContain("x:Name=\"EditBody\"", topBar);