feat(topbar): one decision — [ Build Scenes ][ Record / Go Live ]

Unit A of the top-bar redesign (creator ruling 2026-11-12). The standalone EDIT toggle is gone;
Build Scenes IS edit mode, so "which world am I in?" and "can I edit?" cannot disagree any more.

BarWorld (ViewModels/MainViewModel.TopBar.cs) has two values, Build and Output. Build ships
highlighted at launch and sets EditingActive on; choosing Record / Go Live turns it off. Clicking the
already-highlighted half is deliberately not a no-op: it re-enters that world, which is the only way
back to the layers during a session now that the EDIT button is gone (unit 1 made that legal).

WorldSegmentButton (Themes/Controls.xaml) is a Button, not a ToggleButton, and each half passes its
own flag through Tag to one shared DataTrigger — a ToggleButton would fight the view model and, with a
one-way IsChecked, clear its own binding on first click.

UiShellTests updated where it guarded the old EDIT markup (it now guards the segment). TopBarWorldTests
pins launch state, both transitions, and the re-enter case.

679 pass, 0 warnings. Next: unit B (output switch), unit C (output-dependent action + End labels).
Scope justification: Assets/starting-background.jpg is outside this unit — the creator's replacement
art, still awaiting their visual check.

Themes/Controls.xaml

@@ -1002,6 +1002,33 @@
     <!-- 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 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. -->
+    <Style x:Key="WorldSegmentButton" TargetType="Button">
+        <Setter Property="Background" Value="Transparent"/>
+        <Setter Property="Foreground" Value="{StaticResource Brush.OnAccent}"/>
+        <Setter Property="BorderThickness" Value="0"/>
+        <Setter Property="Cursor" Value="Hand"/>
+        <Setter Property="Height" Value="28"/>
+        <Setter Property="VerticalAlignment" Value="Center"/>
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Button">
+                    <Border x:Name="Face" Background="{TemplateBinding Background}"
+                            CornerRadius="3" Padding="{TemplateBinding Padding}">
+                        <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
+                    </Border>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+        <Style.Triggers>
+            <DataTrigger Binding="{Binding Tag, RelativeSource={RelativeSource Self}}" Value="True">
+                <Setter Property="Background" Value="{StaticResource Brush.Primary}"/>
+            </DataTrigger>
+        </Style.Triggers>
+    </Style>
+
     <Style x:Key="RecordPillToggle" TargetType="ToggleButton" BasedOn="{StaticResource SegmentToggle}">
         <!-- SegmentToggle's template hard-codes a Transparent face and paints the checked state
              itself, so an armed pill needs its own template that honours Background. -->