// RTStress miss (runs in SHADE). Primary miss → sky gradient. Shadow miss
// → the sun is unoccluded, so add the pending direct contribution.
fn miss_main(ray: RayDesc, payload: ptr<function, Payload>) {
    if ((*payload).shadowRay == 1u) {
        rtAccumulate((*payload).color);
        return;
    }
    let t = clamp(ray.direction.y * 0.5 + 0.5, 0.0, 1.0);
    rtAccumulate(mix(vec3<f32>(0.50, 0.62, 0.85),
                     vec3<f32>(0.90, 0.94, 1.00), t));
}