fix: use GroupKFold(source) instead of StratifiedKFold to prevent data leakage (audit #9)

fix: use shap.Explainer for XGBoost 2.x compatibility

fix: exclude 'source' column from XGBoost feature matrix

feat(ml): evaluation pipeline and improved XGBoost training from diagnostic report

feat(ensemble): — XGBoost trained classifier replaces hardcoded signal weights