Home-screen widget goes blank or stale — recents blanked after an iCloud-sync launch ("No recent documents" while the app has plenty), pre-lock titles/thumbnails still visible on the widget while App-Lock is engaged, thumbnails broken after reinstall or device restore, or the widget silently shows nothing because containerURL(forSecurityApplicationGroupIdentifier:) returned nil. Use when building or debugging a WidgetKit extension that shares state with its host app via an App Group (snapshot file, reloadAllTimelines, shared UserDefaults suite).
