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

fix: use project root as snapshot_download local_dir (fix double artifacts/ path)

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