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.

ytLive.Tests/UiShellTests.cs

@@ -39,10 +39,12 @@ public class UiShellTests
         //     line (creator ruling 2026-11-12), and a chevron is the way back.
         Assert.Contains("Data=\"M3,4h8v6H3V4zM13,4h8v6h-8V4zM3,12h8v8H3V12zM13,12h8v8h-8V12z\"", topBar);
         Assert.Contains("Visibility=\"{Binding IsBuildWorld, Converter={StaticResource BoolToVis}}\"", topBar);
-        Assert.Contains("ToolTip=\"Back to Build Scenes\"", topBar);
+        Assert.Contains("Text=\"Output:\"", topBar);
+        Assert.Contains("Text=\"press <esc> to return\"", topBar);
+        Assert.DoesNotContain("ToolTip=\"Back to Build Scenes\"", topBar);   // Esc replaced the chevron
 
         // 2c. The output stage: two labelled switches and the exclusivity promise beside them.
-        Assert.Contains("Text=\"Output\"", topBar);
+        Assert.Contains("Text=\"Output:\"", topBar);
         Assert.Contains("Text=\"YouTube\"", topBar);
         Assert.Contains("Text=\"Local MP4\"", topBar);
         Assert.Contains("Text=\"One output at a time\"", topBar);