Function nonced_leaf_hash 

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

Build the per-leaf nonced freshness hash for a subtree leaf (responder side), reusing the existing audit digest.