Split first, then learn every transformation on train only

Stratified 80/20 split; compute winsor caps, merge/ordinal decisions, selected features and scaler on train; apply to test. Concepts: Leakage-safe preparation order Project A: Check train and test bad rates are both ~30% after stratification. Project B: Chronological split (train ≤ 2017, test 2018); lags and rolling features already use only the past.