Phase 1 of switching processes/qq-list/connections from 3s REST polling to
SSE-pushed snapshots. The bus is a frozen-snapshot fan-out (subscribers added
during a publish() see only the next one, peer unsubscribes mid-fan-out don't
skip survivors) with exception isolation per listener.
Pure pub/sub by design — debounce and snapshot reads belong with the
subscriber (the /api/state/stream handler in phase 3) so different consumers
can coalesce on their own cadence.
8 unit tests, 414/414 core suite green.
DrSmoothl