ConstDisplay a user-friendly error dialog with guidance.
Optionaloptions: ErrorNotificationOptionsBrief toast notification that auto-dismisses.
Show the keyboard-shortcuts help overlay.
Hide the keyboard-shortcuts help overlay.
Display the loading spinner.
Remove the loading spinner.
Clear any displayed error dialog. No-op if none is showing.
Show the persistent scene-identity banner (changed / unreachable).
Hide the scene-identity banner (optionally only a specific kind).
Never warns about a missing backend: this is teardown, not a dropped
message. The app dispose pipeline clears the backend before it destroys
the SceneLoaderManager, so the watchdog's own dispose necessarily lands
after — and "there is no UI left to clear" is the expected state there,
not the misconfiguration warnIfMissing reports.
OptionalonlyKind: "changed" | "unreachable"
Public notifier surface. Methods are stable; backends can come and go without changing call sites.