fix(topbar): the output line REPLACES the segment, it does not sit beside it

Creator correction: choosing Record / Go Live must take the [ Build Scenes ][ Record / Go Live ] dual
button away and put the Output line in its place. It was rendering both, segment first, which read as one
long toolbar instead of two worlds.

The segment is now Build-world only. That also retires the padlock glyph added in 6c5cd2f — with the
segment gone in the Output world there is no state left for it to describe, so the branch is removed
rather than left unreachable.

Because the segment was the only route back to the layers, the Output world gains a left chevron
("Back to Build Scenes"), present for the whole world including while a session runs — otherwise
editing during a show (unit 1) would have had no entry point and the bar would have dead-ended.

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

ytLive.Tests/UiShellTests.cs

@@ -35,11 +35,11 @@ public class UiShellTests
         Assert.Contains("Tag=\"{Binding IsOutputWorld}\"", topBar);
         Assert.DoesNotContain("x:Name=\"EditBody\"", topBar);
 
-        // 2b. Build Scenes carries a layout glyph while it is the active world, and a padlock once
-        //     Record / Go Live is — each lit by its own world flag (creator ruling 2026-11-12).
+        // 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);
-        Assert.Contains("Data=\"M7,10V7a5,5 0 0,1 10,0v3h1", topBar);
-        Assert.Contains("Visibility=\"{Binding IsOutputWorld, Converter={StaticResource BoolToVis}}\"", topBar);
+        Assert.Contains("Visibility=\"{Binding IsBuildWorld, Converter={StaticResource BoolToVis}}\"", topBar);
+        Assert.Contains("ToolTip=\"Back to Build Scenes\"", topBar);
 
         // 2c. The output stage: two labelled switches and the exclusivity promise beside them.
         Assert.Contains("Text=\"Output\"", topBar);