Build-an-X for push notification tests (push notifications, web push, FCM / APNs push messages) across Web Push (RFC 8030 / VAPID), Apple Push Notification Service (APNs), and Firebase Cloud Messaging (FCM) - covers subscription handshake, payload encryption, badge / sound / click-action assertions, expired-subscription cleanup, silent-vs-alert, and topic-vs-targeted routing; also carries the in-app notification test workflow (WebSocket / SSE / Firebase-listener delivery, read-unread state, multi-session fan-out, offline-then-reconnect) in references/in-app.md. Use when authoring tests for any push or in-app notification flow.