风格化：分层光照与 Fresnel 边缘光

void main() {
  vec3 N = normalize(vNormal);
  vec3 L = normalize(vec3(-0.6, 0.8, 1.0));
  vec3 V = normalize(uCameraPos - vWorld);
  float diffuse = max(dot(N, L), 0.0);
  float toon = ceil(diffuse * 3.000) / 3.000;      // 连续 → 分层
  float level = mix(diffuse, toon, 1.000);
  float rim = pow(1.0 - max(dot(N, V), 0.0), 3.000);  // 越靠边越亮
  vec3 color = vec3(0.96, 0.43, 0.30) * (0.2 + 0.8 * level) + vec3(0.55, 0.85, 1.0) * rim * 0.700;
  gl_FragColor = vec4(color, 1.0);
}