Function transfer_receipt_bytes 

pub fn transfer_receipt_bytes(
    recipient_agent_id: &str,
    transfer_id: &str,
    action: &str,
    nonce: &str,
    issued_at_unix: i64,
) -> Result<Vec<u8>>

Canonical bytes signed by the recipient when requesting the blob or acknowledging delivery. Binds the recipient’s identity to the specific transfer_id and a fresh nonce to prevent replay.