Final validation checklist and export

Automated checks (no NaN/inf, all numeric, same columns in train/test, class ratio preserved, no protected features), then save data and the fitted pipeline. Concepts: Pre-modelling QA; persistence Project A: Export X_train/X_test/y_train/y_test, feature list with definitions, the pipeline .joblib and the issue log. Delete the 'TEST CODE TO DELETE' cell. Project B: Export train/test with the time index and the feature list.