Ordered fan-out at scale

Design ordered fan-out to millions of subscribers with per-key ordering. A single source emits updates keyed by entity (e.g. document edits, account-balance events); each update must reach every subscriber of that key, and every subscriber must observe updates for a given key in the exact order they were produced — even though one key may have 5M subscribers and updates arrive at 100K/sec. A few hot keys dominate traffic.