SnifferEngine gains an optional publish_fn hook, invoked after the
dedup + syslog write for traffic-summary events only (tls_session,
tcp_flow_timing, tcp_syn_fingerprint) — intermediate parser artifacts
like tls_client_hello stay off the bus.
The sniffer worker wires get_bus() + a thread-safe shim that marshals
sync calls from the scapy sniff thread back onto the asyncio loop via
run_coroutine_threadsafe. Bus failure at startup degrades cleanly to
publish-off mode; publish failures at runtime never escape the sniff
thread.
