Forecast hyperparameter grid search: defaults already near-optimal

Ran a 27-cell GridSearchCV over XGBoost (n_estimators × max_depth × learning_rate) plus a follow-up min_child_weight/gamma sweep. Holdout AUC improved +0.007. But LOCO median AUC DROPPED -0.003. Best params lose in 7 of 10 most-active countries. The current defaults are at the practical ceiling for this feature set. Future gains require feature engineering, not hyperparams.