Implement tests

Tests fail their job in three quiet ways: the test that passes even when the code under test is commented out (pure ceremony), the test that reaches into internals and shatters on a behavior-preserving refactor (the test broke, not the code), and the test that bundles six unrelated assertions so a failure says "something broke" without saying what. All three are net-negative — they cost maintenance and catch nothing. This guide adds the testing discipline on top of the base implement-task rules. These are conventions the review packet inspects — nothing enforces them at edit time.

This guide is for tests as the deliverable in their own right. Tests written as part of a feature or fix ride inside those guides; a test that already fails non-deterministically belongs to fix-flaky-test.

Rules