Adds CorrelationEngine.correlate_credential_reuse + the
`decnet reuse-correlate` long-running worker. The worker mirrors the
mutator's bus-wake + slow-tick pattern: wakes on credential.captured
and attacker.observed for sub-second latency, falls back to a 60s
poll if the bus is unavailable, and publishes
credential.reuse.detected once per new or grown CredentialReuse row
(group-deduped so a 5-cred reuse doesn't emit 5 partial events).
The web ingester now publishes credential.captured after every
successful Credential upsert; bus + new repo helper
find_credential_reuse_candidates feed the engine pass.
Ingester connects the bus at startup, emits a batch-committed summary
(component/flushed/position) after each successful _flush_batch. Zero-
row flushes are suppressed so the topic stays meaningful.
Complements the collector's per-line system.log publishes: collector
signals ingress, ingester signals DB-persisted progress. Federation
forwarder (worker 8) will subscribe to the batch-committed leaf to
trigger its upstream push.
Bus stays optional: publish_safely swallows failures, get_bus() can
return None, DECNET_BUS_ENABLED=false leaves the ingestion loop fully
functional.
