Function node_publish_payload 

pub fn node_publish_payload(
    path: &str,
    message: &str,
    ts: u64,
    node_id: &str,
) -> Vec<u8> 

Compose the canonical byte payload a node must sign for a substrate.publish.

Layout: b"substrate.publish.node\0" || path || b"\0" || message || b"\0" || ts_le || b"\0" || node_id.

Separate from crate::agent_registry::publish_payload so an actor signature over an ipc.publish payload cannot be replayed as a node signature over a substrate.publish — the domain-separation prefix differs.