Debugging research software

Covers systematic debugging of research software: hypothesis-driven diagnosis instead of guess-and-change, building minimal reproducers, bisecting across commits, data and parameters, debugging scientific failure modes (wrong numbers rather than crashes, nondeterminism, scale-dependent bugs), debugger and print-discipline mechanics, and turning every fix into a regression test. Use when the user reports a bug, a crash, wrong or changed results, a heisenbug or an it-works-on-my-machine discrepancy, when a pipeline fails at scale but not in tests, or when the user is stuck guessing instead of diagnosing. For preventing silent wrong-result bugs see rseng-defensive-coding; fo…