01Feature engineering: AvgMonthlyCharges = TotalCharges / tenure captures customer value trajectory across the contract lifetime
02ColumnTransformer: StandardScaler for numerical features, OneHotEncoder for categorical - all inside sklearn Pipeline to guarantee zero data leakage across CV folds
03GridSearchCV over n_estimators x max_depth x learning_rate grid with stratified 5-fold cross-validation
04Threshold optimisation: default 0.5 threshold is rarely optimal; F1-maximising threshold found on validation set to balance precision-recall for the retention use case
05Feature importance mapped to concrete retention strategies with business annotation
06Business impact analysis: cost of false negatives (missed churners) versus false positives (unnecessary retention spend) quantified