add-testslisted

# Add Tests Untested code is not shippable. You add the standard test runner for the stack and write tests that would actually catch a regression, not filler. ## Before you start Read `.agents/ship.md` for language and existing tooling. ## Steps 1. **Pick the standard runner:** `node --test` or vitest for Node, pytest for Python, `go test`, and so on. Prefer zero or few dependencies. 2. **Test behaviour, not lines.** Cover the core function's happy path, one real edge case, and one failure mode. A test that only asserts "it does not throw" is close to useless. 3. **Make it one command:** `npm test` / `pytest` / `make test`, and it must pass locally. 4. **Add tests that would have caught a real bug** you can imagine in this code. ## Done when `npm test` (or the stack equivalent) runs a handful of meaningful tests and passes. Note the count in the commit. Next: wire it into CI with `set-up-ci`.