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.

MainWindow.xaml.cs

@@ -181,6 +181,14 @@ public partial class MainWindow : Window
         {
             _viewModel.DismissZoomIfActive();
         }
+
+        // Esc leaves a sub-menu (creator ruling 2026-11-12): the Output world is a sub-menu, and its
+        // back chevron is gone in favour of this. The bar says so under the switches.
+        if (e.Key == System.Windows.Input.Key.Escape && _viewModel.IsOutputWorld)
+        {
+            _viewModel.EnterBuildWorldCommand.Execute(null);
+            e.Handled = true;
+        }
     }
 
     private void UpdateTaskbarOverlay()