fix: remaining v3 bugs - TFT n_feat detection, simulate URL, features from artifacts

fix: proper v3 implementation - auto-detect n_feat, fix simulate URL, fix model_count

feat: v3 models - XGBoost R2=0.9866, GradientBoosting R2=0.9860 as default

feat: HF model repo at v1/v2 root; version selector UI with download submenu; artifacts/ out of git

feat: full project — ML simulation, dashboard UI, models on HF Hub