Hosted E2E smoke for PEAR_OBSERVER_STREAM before enabling (merged dark in #393) #405

#393 merged with the flag default OFF. All validation was against a local observer-plane double (real sockets, real ObserverStreamManager) — the full path real broker → hosted Relaycast with the flag ON has not been exercised (matches the PR's own unchecked manual-testing box). Before flipping PEAR_OBSERVER_STREAM on anywhere: one hosted smoke — live #general message with no locally-attached worker arrives via the stream; kill the stream and confirm silent fallback to polling; restart and confirm cursor resume without duplicates (reconcileMessages dedupe).