Migrate a setState StatefulWidget to a Riverpod Notifier in Flutter
A step-by-step move from widget-local setState to a Riverpod 3.x Notifier: classify what actually leaves the widget, write the Notifier, convert to ConsumerWidget, and survive the == filtering, build() re-entry, and autoDispose defaults that bite setState refugees. Tested on Flutter 3.44, Dart 3.x, flutter_riverpod 3.3.2.