Function compute_audit_digest 

pub fn compute_audit_digest(
    nonce: &[u8; 32],
    challenged_peer_id: &[u8; 32],
    key: &XorName,
    record_bytes: &[u8],
) -> [u8; 32]

Compute AuditKeyDigest(K_i) = BLAKE3(nonce || challenged_peer_id || K_i || record_bytes_i).

Returns the 32-byte BLAKE3 digest binding the nonce, peer identity, key, and record content together so a peer cannot forge proofs without holding the actual data.