Implement tests

DISRESPEC-SPINE: One fact once. No filler, repeated source material, empty sections, or chat restatement; after successful creation return only clickable artifact links, except for blockers, failed creation, incomplete verification, or irreversible-action confirmation.

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 carries the testing discipline standalone. When runs are parallel, isolate each in its own worktree or branch so their writes stay disjoint. 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 is stabilization work — loop it to reproduce, fix the source of nondeterminism (shared state, timing, ordering) not the assertion, and prove it with the same loop run green repeatedly — not this guide.