tdd

# TDD ## Purpose Implement one observable behavior at a time through the narrowest public seam: ```text RED: one test fails for the missing target behavior GREEN: the smallest production change makes it pass REFACTOR: improve structure only while green ``` Use this for production features, confirmed bug fixes, and behavior-preserving refactors. Use `diagnose` first when the root cause is unknown. Use `prototype` when the code is disposable design learning. ## Choose A Vertical Slice Before editing production code: - identify the user or caller-visible outcome - identify the public interface and nearest real integration path - inspect adjacent tests and project conventions - select one vertical tracer bullet that can be independently proven Prefer a narrow end-to-end behavior over a broad layer-by-layer implementation. Read `references/interface-design.md` when the public seam is awkward or missing. ## RED Write exactly one test for one behavior. Do not write every imagined test up front. Run the actual test command and confirm: - the test fails before production code changes - the failure is caused by the missing target behavior, not syntax, fixture, environment, or setup failure - the assertion describes public behavior rather than private implementation shape If the test is already green, it is not RED evidence. Correct the seam or demonstrate that the requested behavior already exists before proceeding. Read `references/tests.md` and `references/mocking.md` w...