Adds DQNTrainer::train_fold_from_slices(&[[f64;42]], &[[f64;4]]) so the fold loop in train_baseline_rl passes raw fxcache slices directly, without the caller constructing any Vec<(FeatureVector, Vec<f64>)>. Removes features_to_trainer_format_fast helper (no longer needed) and updates train_dqn_fold to call the new method. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>