使用 Three.js 实现逼真的乌云密布特效，具体要求：

**核心特效**
- 体积云 raymarching
- 内光源闪烁
- 多层噪声叠加
- 消散效果
- 抗锯齿渲染

**场景与光照**
- 全屏 PlaneGeometry
- 自定义 raymarching
- 球体内渲染

**交互与控制**
- 无控制器，纯动画展示

**技术要求**
- Three.js 版本 (ES Module)
- 自定义 ShaderMaterial
- PlaneGeometry 全屏
- 复杂 raymarching 算法

float octavedNoise(vec3 position) {
    vec3 samplePosition = position * 2.;
    float noiseAmount = noise(samplePosition + iTime * vec3(0.0,0.2,0.0));
    samplePosition *= 1.99;
    noiseAmount += noise(samplePosition + iTime * vec3(0.05,-0.37,0.02)) * 0.51;
    noiseAmount /= 1.51;
    return noiseAmount;
}

float density(vec3 position) {
    float baseValue = 1.0 - pow(max(0.0, length(position)), 2.0);
    float noiseAmount = octavedNoise(position);
    return max(0., min(1., baseValue - max(0., noiseAmount * 1.5)));
}

vec4 innerLightPositionAndIntensity() {
    float scaledTime = iTime * 6.1;
    float hashInput = floor(scaledTime) * 0.1;
    if (hash11(hashInput) < 0.8) return vec4(0.); // 大部分时间熄灭
    // ...
    float intensity = sin(fract(scaledTime) * 3.142);
    return vec4(position, intensity);
}

for(int i = 0; i < mainSteps; i++) {
    float localDensity = min(1.0, density(samplePosition) * mainDensityScale);
    if (localDensity > 0.001) {
        // 计算主光源阴影
        // 计算内光源衰减
        transmittance *= (1. - stepDensity);
        if (transmittance < 0.01) break;
    }
    samplePosition += mainStepAmount;
}