Scene-depth volumetric clouds

Reuses Zero's v5 Perlin-Worley density generation and multi-scatter lighting. Converts density to a storage buffer because render-feature passes do not accept 3D textures. Marches at 90% viewport resolution against the active camera and opaque depth, after atmosphere and before lens effects. The 220–3300 m cloud layer drifts in world space, with two procedural erosion octaves for nearby detail. Uses up to 128 adaptive steps, increasing stride with distance and empty-space skipping. The environment module owns its lifetime.