Verify test efficacy with a mutation check: break the code, confirm the test goes red, revert

A newly written test passing tells you the code works now — it does NOT tell you the test would catch a regression. Tests that assert the wrong thing, assert against a re-implemented copy of the logic, or are vacuously true (e.g. asserting "diverged === 0" when nothing was ever compared) pass green while guarding nothing. Writing tests to "complete coverage" without checking efficacy produces a green suite that gives false confidence.