Mutation Testing

Add a third validation layer to the ATDD two-stream testing approach. Acceptance tests verify WHAT, unit tests verify HOW, mutation testing verifies that the tests actually catch bugs.

Core Concept

Mutation testing introduces deliberate bugs (mutants) into source code, then runs the test suite. If tests fail, the mutant is killed (good). If tests pass despite the bug, the mutant survives (test gap found).

Source code → introduce mutation → run tests
                                     ├── tests FAIL → mutant killed ✓
                                     └── tests PASS → mutant survived ✗