Implements CPU-based BRDF LUT generation using the split-sum IBL approximation (Hammersley sampling, GGX importance sampling, Smith geometry with IBL k=a²/2). Wraps the 256×256 Rgba8Unorm LUT in IblResources for GPU upload via wgpu 28.0 API. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>