feat(topbar): three fixed slots — every state changes inside its own slot

Creator correction (2026-11-13): "OpenCode literally inserted Live and Private as two extra flat siblings
inside the outer pill bar, shoving Record to the right and making it look like five ungrouped toolbar
items". Correct — completion states were being added to the BAR instead of to a SLOT.

Now the bar is exactly three slots, and the bar's shape never changes:

  [ Build Scenes ]   [ YouTube ]   [ Record ]

  YouTube:  ▶ YouTube -> [Live | Private | ✕] -> [● Start Live | ✕] (amber: next click goes on air)
            -> [■ Stop Live]
  Record:   ◉ Record  -> [● Start Recording | ✕] -> ● REC hh:mm:ss [■ Stop]
  Build:    enabled only while BOTH action slots are Idle; otherwise disabled at 40% with the tooltip
            "Scene editing is locked while streaming or recording"

State lives in YouTubeSlotState / RecordSlotState, with one RaiseSlotStates() announcing every flag so no
caller can forget one. Esc and the ✕ cancel the slot you are in and nothing else.

This retires the BarWorld model from earlier today, along with TopBarActivityTests (superseded by
TopBarSlotTests, which pins all three flows, the lock, and cancel-to-Idle).

679 pass, 0 failed (the host-hang fallback in f4caea0 counted the per-test lines).
Scope justification: TopBarActivityTests.cs is deleted as part of this unit — it tested the model being
replaced; TopBarSlotTests is its successor.

ViewModels/MainViewModel.Streaming.cs

@@ -87,7 +87,7 @@ public partial class MainViewModel : ViewModelBase
                 OnPropertyChanged(nameof(ShowChatInactiveMessage));
                 OnPropertyChanged(nameof(ShowPreviewPlaceholder));
                 RaiseTopBarModes();
-                RaiseActivityStage();     // IsLive drives ShowOutputStage
+                RaiseSlotStates();     // IsLive drives ShowOutputStage
                 UpdateLiveVisuals();
             }
         }
@@ -118,7 +118,7 @@ public partial class MainViewModel : ViewModelBase
         {
             if (!SetProperty(ref _recordPillOn, value)) return;
             if (value && _onAirPillOn) OnAirPillOn = false; // record-OR-live — arming REC drops ON-AIR
-            RaiseActivityStage();
+            RaiseSlotStates();
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(PrimaryStartKind));
             OnPropertyChanged(nameof(PrimaryStartButtonLabel));
@@ -138,7 +138,7 @@ public partial class MainViewModel : ViewModelBase
             // intent, not reality; the Start face turns this into a sign-in.
             if (!SetProperty(ref _onAirPillOn, value)) return;
             if (value && _recordPillOn) RecordPillOn = false; // record-OR-live — arming ON-AIR drops REC
-            RaiseActivityStage();
+            RaiseSlotStates();
             OnPropertyChanged(nameof(ShowPrimaryStartButton));
             OnPropertyChanged(nameof(PrimaryStartKind));
             OnPropertyChanged(nameof(PrimaryStartButtonLabel));
@@ -171,7 +171,7 @@ public partial class MainViewModel : ViewModelBase
             OnPropertyChanged(nameof(EndButtonText));
             OnPropertyChanged(nameof(ShowTestButton));
             RaiseTopBarModes();
-            RaiseActivityStage();         // Start Recording <-> Stop Recording, and the REC clock
+            RaiseSlotStates();         // Start Recording <-> Stop Recording, and the REC clock
             UpdateSessionTimer();
         }
     }