<objective Write unit tests that fail when the code is wrong and pass when it is right — nothing weaker. A test that mocks every collaborator stays green while the integration is broken; the doubles taxonomy below stops that. A typo (or the plural , which Jest silently ignores) lets 40%-covered code ship on a green pipeline; the config and Verification sections below make the gate actually fire. This skill covers Jest, Vitest, and pytest: doubles, coverage gating, snapshots, fake timers, and mutation testing as a behavior check on top of coverage. </objective --- Discovery Questions Check fir…