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.

Before you start, open references/task-template.md and copy it into your task file — it is the session frame for this work; fill it in as you go (don't reconstruct the structure from memory). It scaffolds the coverage gap, the test-case table keyed by behavior, the placement table, the flip evidence, and the self-review. The task packet itself uses the kit's task template.