Implements deferred_lighting.wgsl for the fullscreen lighting pass: reads G-Buffer textures, runs the identical Cook-Torrance BRDF functions (distribution_ggx, geometry_smith, fresnel_schlick, attenuation, etc.) from pbr_shader.wgsl, computes multi-light contribution with PCF shadow and IBL, then applies Reinhard tonemapping and gamma correction. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>