feat(topbar): three activities, always visible — build, stream, record

Creator ruling 2026-11-13: "the app does three things. builds. records. live-streams." So the bar holds
three buttons that never disappear; the active one is lit and runs its own flow in place, and the other
two dim but stay clickable, so there is no state where the creator cannot see where they are or how to
step back.

  Build    [Build Scenes]  -> editing on, nothing to start
  YouTube  [Live][Private] -> [Start] -> [Stop]     (Private = the private-visibility test stream)
  Record   [Start Recording] -> [● REC hh:mm:ss][Stop Recording]

Arming and starting are separate presses on purpose (creator): Start is the last chance to tell someone
"check you are ready". Collapsing them would save one click and lose that prompt.

This replaces the two-world model (BarWorld) and its output stage outright, including the chips added
earlier today. The old pills remain the underlying state (OnAirPillOn/RecordPillOn) so the start, stop and
step-down paths are untouched; the activity only decides what the bar shows.

Closing the app no longer waits 6 s for a session that cannot stop: it waits ~2 s, then clears the state
and closes anyway. A window that refuses to close is worse than a stale flag — and that refusal is what
hung the suite in ZoomBoxInteractionTests, which sets IsRecording and closes a real window.

Test: TopBarActivityTests pins all three flows, the arm-then-start split, and that stepping back to Build
forgets the YouTube choice instead of carrying it. TopBarWorldTests is retired with the model it tested.

MainWindow.xaml.cs

@@ -204,9 +204,9 @@ public partial class MainWindow : Window
 
         // 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)
+        if (e.Key == System.Windows.Input.Key.Escape && _viewModel.Activity != ViewModels.TopActivity.Build)
         {
-            _viewModel.EnterBuildWorldCommand.Execute(null);
+            _viewModel.SelectBuildActivityCommand.Execute(null);
             e.Handled = true;
         }
     }