Improve IBLSheenBRDF approximation accuracy

[mrdoob]

Related issue: #32356

Description

Replaces the piecewise IBLSheenBRDF curve-fit with an improved unified formula.

Before	After

Changes:

• 5x better approximation accuracy (62% → 13% average error vs ground truth)
• Removes branch (roughness < 0.25 check)
• Removes log() call
• Simpler and more efficient

The new coefficients were optimized by numerically integrating the Charlie sheen BRDF over the hemisphere and fitting across the full roughness range.

(Made using Claude Code with Opus 4.5)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.29 83.05	350.27 83.05	-22 B +4 B
WebGPU	613.67 170.41	613.66 170.42	-10 B +1 B
WebGPU Nodes	612.27 170.14	612.26 170.14	-10 B +3 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.28 117.83	482.26 117.84	-22 B +2 B
WebGPU	684.81 186.19	684.8 186.19	-10 B +4 B
WebGPU Nodes	634.64 173.38	634.64 173.38	+0 B +0 B