Function compute_message_hash 

pub fn compute_message_hash(
    payload: &AttestationPayload,
    signers_bitmap: [u8; 32],
) -> [u8; 32]

Compute the EVM-compatible attestation message hash.

Matches Validator._constructMessage in the EVM reference implementation:

keccak256(abi.encodePacked(
    MESSAGE_PREFIX, value, sourceId, registryVersion, signaturesRequired,
    canonicalTimestamp, signersBitmap
))