Function write_enqueue_source 

pub fn write_enqueue_source(
    repo_root: &Path,
    mission_id: &str,
    producer: &str,
    external_ref: &str,
) -> Result<EnqueueSource>

Persist an external producer binding atomically. Callers must do this before enqueue so even a sibling dispatcher that claims immediately cannot erase the only mission-to-producer join.