Property-Based Testing and Mutation Testing

Deferred to Phase 2.25: Mutation testing runs ONCE per feature as final quality gate at orchestrator Phase 2.25 (after all steps complete). Do NOT run mutation testing during inner TDD loop.

Property-Based Testing (PBT)

Instead of examples ("given X, expect Y"), write properties ("for all valid inputs, condition Z holds"). Framework generates hundreds/thousands of inputs checking property. Dramatically expands test coverage.

Property Patterns

1. Invariants: "for all inputs, condition holds" (sorted list is ordered, balance >= 0)
2. Roundtrip: "encode then decode = original" (serialize/deserialize, compress/decompress)
3. Oracle: "compare against reference implementation" (optimized vs correct-but-slow)
4. Metamorphic: "different operations, same result" (add(a,b)==add(b,a), filter can't increase size)

Shrinking

When property fails, framework auto-finds minimal failing input. Dramatically accelerates debugging. Algorithm: find failing input -> try simpler variants -> if still fails, use as new candidate -> repeat.