Function dispatch_untrusted_fan_in 

pub async fn dispatch_untrusted_fan_in(
    engine: &Arc<Mutex<SopEngine>>,
    audit: &SopAuditLogger,
    source: SopTriggerSource,
    topic: Option<&str>,
    payload: Option<&str>,
    dedup: Option<(String, bool)>,
) -> Vec<DispatchResult>

Headless fan-in chokepoint for untrusted external events (channel messages, AMQP deliveries, …): caps oversized topic/payload at the configured untrusted_payload_max_bytes (with an explicit truncation marker), stamps the event, dispatches it against loaded SOP triggers, and audits the results. Callers should gate on SopEngine::wants_source first to skip the work when no SOP listens.