WebGPU製の手続き型レースゲーム：AIライバルと海沿いコース

Build a complete, production-quality browser racing game called Spline Rush using the latest Three.js (WebGPURenderer + TSL where possible). 100% procedural: no external models, textures, audio files or fonts. Everything generated in code at runtime.

CORE GAME
- 6 unique tracks with elevation, banking, tunnels, hairpins, named corners and distinct biomes (coastal day, mountain dusk, desert sunset, forest rain, night city neon, high-speed oval).
- Championship mode (qualifying + 3 races), Time Trial with ghosts, Quick Race.
- 8 AI opponents with personality, racing line, braking points, overtaking and defending.
- Best lap records, sector times, live event feed, replay camera.
- Garage: 5 parametric cars with clearcoat + metal-flake paint, panel gaps, working lights, animated suspension, damage states.

GRAPHICS TARGET (Ultra, worthy of RTX 5090 at 4K)
Renderer: THREE.WebGPURenderer. Physically based pipeline.
Lighting:
- Rayleigh/Mie physically based sky + starfield + moon + dynamic sun that drives a full day/night cycle.
- Cascaded shadow maps (4 cascades, stable texel snapping, high-res).
- IBL via PMREM updated with time of day.
- Volumetric fog + god rays + heat haze.
Materials:
- MeshPhysicalMaterial / TSL nodes: clearcoat, anisotropy, transmission on glass, metal-flake paint, wet-road shader that reacts to rain.
Post-processing chain (RenderPipeline / TSL or postprocessing library):
GTAO or high-quality SSAO → SSR → bloom (Karis) → motion blur (velocity) → DOF → god rays → auto-exposure → color grading + film grain + vignette → SMAA or TAA.
Effects:
- GPU particle pools: tyre smoke, sparks, dust, rain spray, grass/gravel kick-up, heat distortion.
- Skid marks that persist and fade.
- Dynamic wetness and puddle reflections when raining.

PHYSICS & FEEL
- Fixed-step 120 Hz simulation.
- Raycast or strut suspension, load transfer, combined-slip tyres, ABS/TC, surface types (asphalt, kerb, grass, gravel, wet).
- Camera: cinematic chase + hood + onboard with motion and collision shake.

AUDIO
- Fully synthesised Web Audio: multi-layer engine by RPM/load, wind, tyre screech, kerb rumble, crowd, dynamic music.

QUALITY SYSTEM
- Presets: Low / Medium / High / Ultra.
- Ultra assumes RTX 5090-class GPU: 4K, high shadow maps, max particles, all post effects on, no aggressive LOD.
- Adaptive quality that can drop effects if frame time exceeds target.

Start with a playable first version (one track, one car, basic lighting), then iterate feature-by-feature exactly as requested. Keep everything in a single clean HTML/JS (or Vite) project that runs locally. Comment major systems. Make it look expensive, not cute.