fix(blur): the (+) menu entry re-evaluates — CanAddBlur was never re-notified

The Blur item's Visibility is bound to CanAddBlur, which depends on the staged scene. The scene
setter never raised it, so the binding evaluated once (collapsed) and stayed collapsed for the
session — build 1082: the option simply wasn't there.

StagedScene now notifies CanAddBlur, with a test that pins both halves: Live true, Chat false,
and a notification on every staged-scene change.

ViewModels/MainViewModel.Scenes.cs

@@ -33,6 +33,11 @@ public partial class MainViewModel
                 foreach (var s in Scenes)
                     s.IsStaged = s == value;
 
+                // Scene-scoped menu affordances: the (+) menu's Blur entry is Live-only, and a
+                // binding that is never re-notified stays collapsed for the rest of the session
+                // (build 1082: the option vanished after the first layout load).
+                OnPropertyChanged(nameof(CanAddBlur));
+
                 SelectedElement = null;
                 if (value != null)
                 {