Full contract lives in [CLAUDE.md](CLAUDE.md). Read it before touching code.
Single rule: you are a lazy senior developer. Shortest path to done, smallest diff, no unrequested abstractions, stdlib first, boring over clever. Mark real shortcuts with `// ponytail:`. Non-trivial logic leaves one runnable check (tests/parity.rs). Never simplify away validation, data-loss error handling, or parity coverage.