PointShadowNode: Refactor to use native cube depth texture

[mrdoob]

Related issue: #32303 #32365

Description

Refactors point light shadows from the 2D texture atlas approach (with cubeToUV mapping) to using native cube depth textures.

Changes

• Replace 2D atlas viewport layout with native CubeDepthTexture rendering
• Remove cubeToUV helper and viewport calculations
• Add cubeDepthTexture type support to node builders
• Add samplerCubeShadow (GLSL) and texture_depth_cube (WGSL) shader types
• Update CubeTextureNode UV handling for depth texture sampling
• Add cube face framebuffer binding in WebGL backend

Works on both WebGPU and WebGL backends.

(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	350.29 83.05	+0 B +0 B
WebGPU	612.89 170.29	613.1 170.13	+210 B -165 B
WebGPU Nodes	611.49 170.01	611.7 169.85	+210 B -156 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.23 117.82	482.23 117.82	+0 B +0 B
WebGPU	683.78 186	684.25 185.94	+470 B -55 B
WebGPU Nodes	633.61 173.18	634.08 173.15	+470 B -36 B

[mrdoob]

@sunag Let me know if you want me to do any changes.