WebGLRenderer: Add clearcoat energy conservation

[mrdoob]

Related issue: #XXXX

Description

Implements energy conservation for clearcoat materials by attenuating the base layer based on clearcoat reflectance, matching Blender and other PBR renderers.

Changes

• Base layer now correctly attenuated by (1 - clearcoat * Fresnel)
• Applied to direct lighting, indirect diffuse, and indirect specular

Visual Impact
Materials with clearcoat will appear more physically accurate. High clearcoat values (>0.5) will be darker than before, which is correct - the old implementation was too bright and violated energy conservation.

References

• Disney BRDF 2015 paper
• Blender's implementation in gpu_shader_material_principled.glsl
• glTF KHR_materials_clearcoat specification

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	354.98 86.28	356.5 86.47	+1.52 kB +188 B
WebGPU	609.42 170.96	609.42 170.96	+0 B +0 B
WebGPU Nodes	608.02 170.69	608.02 170.69	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	486.63 121.03	488.15 121.21	+1.52 kB +184 B
WebGPU	678.38 186.27	678.38 186.27	+0 B +0 B
WebGPU Nodes	620.38 169.54	620.38 169.54	+0 B +0 B