Whether the relay pool is connected and able to handle requests.
Publish event to relayUrls. May return a per-relay success map; a
void/missing return is treated as optimistic success on every target.
Subscribe to filters on exactly relayUrls. The callback receives each
matching event or the literal 'EOSE' once stored events are exhausted.
Returns a handle to cancel the subscription.
Relay pool contract the router drives. Implementors adapt their pool library (nostr-tools SimplePool, applesauce-relay, etc.). Unlike the lower-level relay NAP pool, both methods take an explicit relay-URL set so the router controls outbox routing and can attribute events to the relay they arrived on.