PMREMGenerator: Replace separable blur with spiral blur on both renderers

[mrdoob]

Description

This PR replaces the separable blur (latitudinal/longitudinal) in PMREMGenerator with a spiral kernel that importance-samples the Gaussian, ports the same approach to the WebGPU PMREMGenerator so both renderers produce matching output, and optimizes the internal vertex shader.

Changes

• Spiral Blur: Golden Angle spiral kernel, isotropic on the sphere. Replaces the latitudinal/longitudinal approximation, which was "least accurate at the poles" by its own admission and needed a poleAxis special case.
• Importance Sampling: Samples are placed by stratified inverse-CDF sampling of the Gaussian, so every sample carries equal weight (~2x the effective sample count of a weighted kernel at the same cost). A sin(theta) / theta term corrects planar sample density to solid angle.
• Split Sigma: _blur performs two passes at sigma / sqrt(2), composing to the requested sigma while squaring the effective sample count.
• Robustness: The kernel truncates at three standard deviations or the antipode, and sigma is clamped to PI. Large sigmas now produce the requested blur; the previous kernel was capped at 20 pixels of reach and silently under-blurred beyond sigma ≈ 0.04 at 256 (e.g. sigma = 1.0 produced only ~0.12 radians of blur).
• WebGPU: The PMREMGenerator of WebGPURenderer received the same port (TSL blur in PMREMUtils.js plus the baked outputDirection attribute), removing the last divergence between the two generators for fromScene().
• PMREMUtils: The internal blur function is renamed to sphericalGaussianBlur (with new parameters). Nothing is added to the TSL namespace (PMREMUtils is no longer exported there since TSL: Do not export PMREMUtils #33986).
• Shader Optimization: The outputDirection attribute is precomputed in JavaScript, removing the faceIndex attribute and the branching getDirection logic from the vertex shader, plus the now-unused uv attribute.
• Cleanup: Removed poleAxis, the CPU Gaussian weight table, dTheta, the vestigial sigmas array and EXTRA_LOD_SIGMA. Blur uniforms reduced from 7 to 3. GGX_SAMPLES aligned to 256 on both renderers (was 512 on WebGPU; 0.03% of pixels differ, halves the prefilter cost).

Verification

• fromScene( new RoomEnvironment(), 0.04 ) is pixel-equivalent to dev on both renderers (mean diff 0.7/255, under 0.4% of pixels above 5/255).
• WebGPURenderer and WebGLRenderer now agree on fromScene() output (0.04% of pixels differ at sigma = 0.04); WGSL and GLSL node backends match at 0.01%.
• The equirectangular path through the reworked vertex attribute is pixel-identical (0.01%).
• Total fromScene() time is unchanged (sub-millisecond either way; the blur does 40 texture taps per pixel vs 78 before).
• sigma = 0 and sigma = Infinity degrade gracefully (copy and near-uniform blur respectively).

(Made using https://jules.google/, https://antigravity.google/ and Claude Code)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	366.73 87.07	365.5 86.6	-1.23 kB -473 B
WebGPU	677.88 187.81	676.2 187.18	-1.67 kB -633 B
WebGPU Nodes	675.92 187.49	674.24 186.86	-1.67 kB -629 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	510 124.11	508.77 123.63	-1.23 kB -473 B
WebGPU	758.56 204.67	756.88 203.98	-1.69 kB -686 B
WebGPU Nodes	708.34 192.16	706.65 191.5	-1.69 kB -659 B