robbik/nucleus8583 — reverse-engineered prompt

Build me a small Java library for working with ISO 8583 financial messages. I want it to make it easy to define a message format, set fields like the message type, bitmap, processing code, amount, dates, trace number, and response code, then pack the message into the wire format and unpack it back into useful Java objects.

Keep the main part clean and reusable, with a separate optional area for any OIM related code if that makes sense. Add simple examples that show a purchase request, a response, and a failed parse case. Please include tests so I can trust that packing and unpacking the same message gives the same result.

There isn’t much README guidance, so keep the scope focused on a practical ISO 8583 toolkit rather than a full payment switch. Look up current ISO 8583 conventions online if you need to, but keep the API simple enough for someone new to the standard to use.