Function checkpoint_signing_bytes 

pub fn checkpoint_signing_bytes(
    key_id: &str,
    seq_high: Seq,
    head: &[u8; 32],
    time_ms: EpochMs,
) -> Vec<u8> 

Canonical checkpoint signed payload.

The layout is DOMAIN ‖ key_id_len(u16 BE) ‖ key_id ‖ seq_high(u64 BE) ‖ head(32) ‖ time_ms(i64 BE).