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This retires the BarWorld model from earlier today, along with TopBarActivityTests (superseded by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.

ytLive.Tests/UiShellTests.cs

@@ -32,8 +32,8 @@ public class UiShellTests
         Assert.Contains("Text=\"Build Scenes\"", topBar);
         Assert.Contains("Text=\"YouTube\"", topBar);
         Assert.Contains("Text=\"Record\"", topBar);
-        Assert.Contains("Tag=\"{Binding IsBuildActivity}\"", topBar);
-        Assert.Contains("Tag=\"{Binding IsYouTubeActivity}\"", topBar);
+        Assert.Contains("IsEnabled=\"{Binding IsBuildScenesEnabled}\"", topBar);
+        Assert.Contains("Command=\"{Binding OpenYouTubeCommand}\"", topBar);
         Assert.DoesNotContain("x:Name=\"EditBody\"", topBar);
 
         // 2b. The segment is Build-world only: choosing Record / Go Live REPLACES it with the output
@@ -51,8 +51,8 @@ public class UiShellTests
         // 2c. The output stage: two labelled switches and the exclusivity promise beside them.
         
         Assert.Contains("Text=\"YouTube\"", topBar);
-        Assert.Contains("Command=\"{Binding SelectYouTubeActivityCommand}\"", topBar);
-        Assert.Contains("Command=\"{Binding SelectRecordActivityCommand}\"", topBar);
+        Assert.Contains("Command=\"{Binding OpenYouTubeCommand}\"", topBar);
+        Assert.Contains("Command=\"{Binding OpenRecordCommand}\"", topBar);
 
         // 2d. The bar carries NO action button: the output line is the bar's whole middle
         //     (creator ruling 2026-11-12 — the action is placed elsewhere, on instruction).