Stream a run's events

Stream a run's events: its durable history first, then live events. Live events carry an SSE id (a stream position); history and the reset frame carry none, and heartbeats carry the current position. Reconnect with the last id you saw as Last-Event-ID to resume exactly after it, late events included; a position too old to resume sends the run's full history again. since_global_seq (or a numeric Last-Event-ID) starts the history after that sequence. An event can arrive more than once across reconnects: dedupe on event_id. Within one connection each event is sent once.