fn miss_main(ray: RayDesc, payload: ptr<function, Payload>) {
    if ((*payload).shadowRay == 1u) {
        // Shadow ray escaped to infinity — the sun is visible from the
        // origin, so the surface there should pick up full direct light.
        // raygen reads color.x as the visibility coefficient.
        (*payload).color = vec3<f32>(1.0);
        return;
    }

    // Primary miss: cheap two-stop sky gradient. (*payload).hit stays 0
    // so raygen knows to skip the lighting path and just use this color.
    let t = clamp(ray.direction.y * 0.5 + 0.5, 0.0, 1.0);
    let sky    = vec3<f32>(0.45, 0.65, 0.95);
    let zenith = vec3<f32>(0.95, 0.85, 0.65);
    (*payload).color = mix(sky, zenith, t);
}