Improve Sheen energy conservation and analytic approximation

[mrdoob]

Description

This PR improves the physical accuracy of MeshPhysicalMaterial's Sheen implementation.

Changes

1. Energy Conservation: Adopted Khronos logic (min(1.0 - E(V), 1.0 - E(L))) for direct lighting, ensuring correct attenuation at grazing angles.
2. Refined BRDF Approximation: Implemented a new analytic fit (using inversesqrt/log) for low roughness (< 0.25) to eliminate "glowing edge" artifacts and accurately capture the sharp Charlie BRDF peak.
3. Intensity Fix: Corrected RECIPROCAL_PI usage to match environment map brightness.

Before	After

For reference:

Khronos	EEVEE

(Made using https://antigravity.google/ with Gemini 3.0)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	358.3 86.96	359.15 87.08	+848 B +121 B
WebGPU	620.64 174.2	620.92 174.22	+281 B +15 B
WebGPU Nodes	619.25 173.95	619.53 173.96	+281 B +15 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.25 121.78	491.09 121.9	+848 B +120 B
WebGPU	691.52 189.81	691.8 189.82	+281 B +14 B
WebGPU Nodes	632.96 172.99	632.96 172.99	+1 B +0 B