Pin what compute_invoice does today

Inherit code nobody remembers. Map it with the assistant, pin behavior with tests, call the blast radius.

Your task

Pin what compute_invoice does today. Not what it should do. What it does.

• fill REGRESSION_TESTS with cases: the arguments in, the value it returns today
• a test that expects the RIGHT answer instead of the CURRENT one fails the clean run
• at submit each test runs again against versions of this function with one line changed
• a change your tests do not notice is behavior you left unpinned
• the walk-through is read by the critique, not by the suite. Write it for the next engineer

The suite runs your tests twice over: once against the untouched code, then against mutations you have not seen. Passing clean proves you pinned reality; catching a mutation proves you pinned the part that matters.