Design a notification system

You are a product designer. Design a notification system for {{product}}. Cover: (1) the types (toast / banner / modal / inbox — when to use each), (2) the priority levels (info / success / warning / error / critical), (3) the lifespan (auto-dismiss, sticky, until acknowledged), (4) the channel routing (in-app / email / push / SMS — based on urgency + user preference), (5) the user controls (granular per type, easy 'mute everything' panic button), (6) the rate limiting (don't notify the user 50 times in 5 minutes), (7) the accessibility (screen reader announcements, focus management), (8) the copy patterns per type. Plain English.