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.

docs/ai/ui-surface.md

@@ -151,7 +151,10 @@ Placement math itself is covered by `WindowPlacementTests` (pure function, no HW
   leads the centre cluster and stays present in every state; clicking the highlighted half re-enters it,
   which is how a creator gets back to the layers during a session. `WorldSegmentButton`
   (`Themes/Controls.xaml`) is a Button, not a ToggleButton — the highlight follows the view model, and a
-  click must never be what sets it.
+  click must never be what sets it. The segment exists **only** in the Build world: choosing Record / Go
+  Live **replaces** it with the output line, and a left chevron (`Back to Build Scenes`) is present for
+  the whole Output world, running included — the way back to the layers, and the reason the bar has no
+  dead end.
 - **The output stage** (`ShowOutputStage`) is the idle cluster and lives only in the Record / Go Live
   world while nothing runs: `Output [▶ YouTube] [● Local MP4] One output at a time`, each switch carrying
   its glyph, with **Local MP4 armed at launch** (`RecordPillOn` starts true — it did not before). The two