GossipSub heartbeat procedure (docs/11-pubsub.md).

The heartbeat runs periodically and performs: 1. Mesh maintenance: prune negative-score, fill undersubscribed, trim oversubscribed 2. Fanout maintenance: expire old, fill undersubscribed 3. Gossip emission: send IHAVE to non-mesh peers, rotate cache 4. Score decay: decay all counters for all peers 5. Seen cache cleanup: remove expired entries 6. Heartbeat counter increment (for opportunistic graft timing)