TSL: Auto-cache atomic* nodes

[sunag]

Related issue: #30971

Description

This avoids double-adding by adding assign() multiple times without using .toConst() or .toVar().

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.33 78.33	336.33 78.33	+0 B +0 B
WebGPU	547.22 151.68	547.3 151.71	+82 B +33 B
WebGPU Nodes	546.57 151.52	546.66 151.55	+82 B +33 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.78 112.28	465.78 112.28	+0 B +0 B
WebGPU	621.61 168.13	621.69 168.15	+82 B +20 B
WebGPU Nodes	576.48 157.44	576.56 157.46	+82 B +20 B