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.
