WebGLRenderer: Fix specular occlusion for smooth metals

[mrdoob]

Related issue: #32097

Description

Fixes over-darkening of smooth metallic surfaces in occluded areas by gradually disabling specular occlusion for low roughness values (< 0.1). Previously, the specular occlusion formula was too aggressive on mirror-like surfaces, causing them to go black in occluded areas. This is physically incorrect since smooth surfaces have highly directional reflections that are less affected by local geometry occlusion. The fix uses a smoothstep to blend between full specular occlusion (roughness ≥ 0.1) and no occlusion (roughness ≤ 0.01), following Filament's approach. Also clarifies in comments that multi-bounce AO is a diffuse-only effect (the implementation was already correct).

https://raw.githack.com/mrdoob/three.js/multibounce-ao-simple/examples/index.html

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.11 84.81	350.55 84.96	+437 B +149 B
WebGPU	604.15 169.43	604.15 169.43	+0 B +0 B
WebGPU Nodes	602.76 169.19	602.76 169.19	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	481.77 119.56	482.21 119.71	+437 B +150 B
WebGPU	673.57 184.88	673.57 184.88	+0 B +0 B
WebGPU Nodes	615.56 168.12	615.56 168.12	+0 B +0 B