All 7 remaining ML models now support SafeTensors serialization:
- Ridge, Lasso, ElasticNet (linear_model)
- DecisionTreeClassifier, RandomForestClassifier (tree)
- KMeans (cluster)
- StandardScaler (preprocessing)

Total Impact:
- +85 comprehensive SafeTensors tests
- All 502 tests passing
- Cross-platform model deployment (Rust ↔ Python ↔ JS)
- HuggingFace ecosystem compatibility

Technical Achievements:
- Tree serialization via pre-order traversal
- Ensemble serialization with index prefixes
- Matrix serialization with shape metadata

GitHub Issue: #8