Function derive_master_key 

pub(super) fn derive_master_key(
    algorithm: &str,
    iterations: u32,
    salt: &[u8],
    username: &str,
    password: &str,
    account_key: &AccountKey,
) -> Result<[u8; 32], OnePasswordError>

Derives the 32-byte master unlock key (kid "mp"), dispatching on the algorithm prefix the way the web client’s Auk.deriveKdfBytes does.