Environment reflection compatibility

Restores the reflection-probe helpers included as zero::environment by this engine build's PBR module. The existing built-in pbr_shading/shader.wgsl supplied the probe selection, priority blending, sky sampling and box/sphere parallax functions. A gather wrapper reports coverage for the PBR ambient fallback. Environment boot registers the literal include key because asset aliases normalize it to @zero::environment. Remove this compatibility module when the engine ships its missing module.