feat(topbar): Output:, no chevron, and Esc returns

Creator ruling 2026-11-12, the parts that were unambiguous:

- The back chevron is gone from the left of the output line.
- "Output" gains a colon and takes the same 11 px muted style as "One output at a time".
- "press <esc> to return" sits centred under the switch pair.
- Esc actually returns from the Output world (MainWindow_PreviewKeyDown -> EnterBuildWorldCommand).
  Without this the bar would have lost its only route back, since the chevron was that route.

UiShellTests guards all four: the colon, the hint, the absence of the chevron tooltip, and the muted
style class of both labels.

680 pass, 0 warnings. The switches' post-click behaviour is still awaiting the creator's mock
description, so this commit deliberately leaves the pill pair alone.

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

Controls/TopBar.xaml

@@ -107,23 +107,13 @@
             <!-- 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
                  family — segments host, Start/Go Live, and Test all sit at 32. -->
-            <!-- The Output world REPLACES the segment (creator ruling 2026-11-12): choosing Record / Go
-                 Live takes the dual button away and puts the output line in its place. This chevron is
-                 the way back to Build Scenes — present for the whole world, running or not, so editing
-                 during a show (unit 1) still has an entry point and the bar has no dead end. -->
-            <Button Style="{StaticResource IconButton}" Width="28" Height="32" Margin="0,0,6,0"
-                    Command="{Binding EnterBuildWorldCommand}" ToolTip="Back to Build Scenes"
-                    Visibility="{Binding IsOutputWorld, Converter={StaticResource BoolToVis}}">
-                <Path Data="M15.41,7.41L14,6l-6,6 6,6 1.41,-1.41L10.83,12z" Width="14" Height="14"
-                      Stretch="Uniform" Fill="{StaticResource Brush.OnSurface}"
-                      HorizontalAlignment="Center" VerticalAlignment="Center"/>
-            </Button>
             <!-- THE OUTPUT LINE (creator ruling 2026-11-12): which single output. Nothing else
                  belongs in it — the action is not part of this row. -->
             <StackPanel Orientation="Horizontal" VerticalAlignment="Center"
                         Visibility="{Binding ShowOutputStage, Converter={StaticResource BoolToVis}}">
-                <TextBlock Text="Output" FontSize="12" FontWeight="Bold" VerticalAlignment="Center"
+                <TextBlock Text="Output:" FontSize="11" VerticalAlignment="Center"
                            Foreground="{StaticResource Brush.OnSurfaceVariant}" Margin="0,0,8,0"/>
+                <StackPanel Orientation="Vertical" VerticalAlignment="Center">
                 <Border Background="{StaticResource Brush.SurfaceContainerLowest}" CornerRadius="4" Padding="2" Height="32" VerticalAlignment="Center">
                     <StackPanel Orientation="Horizontal">
                         <ToggleButton Style="{StaticResource RecordPillToggle}"
@@ -150,6 +140,10 @@
                         </ToggleButton>
                     </StackPanel>
                 </Border>
+                <!-- Esc is the way back now that the chevron is gone (creator ruling 2026-11-12). -->
+                <TextBlock Text="press &lt;esc&gt; to return" FontSize="10" HorizontalAlignment="Center"
+                           Foreground="{StaticResource Brush.OnSurfaceVariant}" Margin="0,2,0,0"/>
+                </StackPanel>
                 <TextBlock Text="One output at a time" FontSize="11" VerticalAlignment="Center"
                            Foreground="{StaticResource Brush.OnSurfaceVariant}" Margin="8,0,0,0"/>
             </StackPanel>