Build a playable Mario Kart-style 3D kart racer in the browser (Three.js, single page, no external assets). A colorful track with jumps, banked turns and boost pads; 5 AI rivals that actually race you; item boxes with banana peels, homing shells and mushroom boosts; drifting with sparks and a mini-turbo; lap counter, live positions and a finish podium. It should feel fast and chaotic the moment it loads.