Build-an-X workflow for testing real-time in-app notifications delivered over WebSocket (RFC 6455) or Server-Sent Events (WHATWG SSE spec), Firebase Realtime Database / Firestore listeners, and notification center read/unread state - covers fan-out to multiple sessions, offline-then-reconnect delivery, and ordering guarantees. Distinct from email, SMS, push, and webhook channels. Use when authoring tests for any notification that appears inside a connected web or mobile app UI without leaving the application.