ISO 9564 PIN Block

Builds the cleartext PIN block exactly as defined by ISO 9564-1: format 0/3 XORs the PIN field against the PAN field; format 1 uses random padding with no PAN. The output is the 16-hex-digit cleartext block — the same value you'd encrypt under a PIN encryption key before sending it to a HSM. No actual encryption is performed and nothing is sent over the network.