Add AutoML module with type-safe hyperparameter optimization

Phase 1 implementation per automl-specification.md:
- SearchSpace with Poka-Yoke enum-based parameter keys
- RandomSearch and GridSearch optimizers
- 6 parameter enums (RandomForest, GradientBoosting, KNN, etc.)
- 28 tests (19 unit + 9 property tests)
- Add ranking metrics (Hit@K, MRR, NDCG)
- Enhance aprender-shell validation