TSL: Support primitive values in custom struct constructors

[sunag]

Related issue: #34085

Description

This PR fixes an issue where instantiating custom structs in TSL using primitive numeric values (e.g., CustomColor( 0.0, 0.0, 1.0 )) was not supported or failed to initialize the struct's fields correctly.

// Define a custom struct type
const CustomColor = struct( { r: 'float', g: 'float', b: 'float' } );

// Fix primitive values passed to a struct
const myColor = CustomColor( 0.0, 0.0, 1.0 );

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.88 86.71	365.88 86.71	+0 B +0 B
WebGPU	685.67 189.48	685.71 189.5	+42 B +20 B
WebGPU Nodes	683.65 189.16	683.7 189.18	+42 B +21 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	510.55 124.02	510.55 124.02	+0 B +0 B
WebGPU	764.04 205.54	764.06 205.54	+22 B +2 B
WebGPU Nodes	713.54 192.96	713.56 192.96	+22 B -2 B

[github-actions]

🖼️ E2E screenshot tests

❌ 1 example(s) failed (full artifacts).

Example	Expected	Actual	Diff
webgpu_animation_retargeting