TSL: Generate native WGSL switch statement (#33548)

[ShiKirill]

Related issue: #33548

Description

Switch() / Case() / Default() previously desugared into a nested if / else chain on every
backend. This adds a dedicated SwitchNode that emits a native WGSL switch statement on the
WebGPU backend, while the WebGL backend keeps the equivalent if / else if / else fallback (GLSL
behaviour unchanged).

• multi-value cases (Case( 1, 2, 3, … )) generate case 1, 2, 3:
• a default clause is always emitted on WGSL (required by the spec)
• selectors are coerced to the integer (i32 / u32) type of the switch expression

Verified identical rendering on both the WebGPU and WebGL backends for single/multi-value cases,
with and without a Default().

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.63 86.86	365.63 86.86	+0 B +0 B
WebGPU	667.42 185.02	668.77 185.47	+1.35 kB +441 B
WebGPU Nodes	665.47 184.71	666.82 185.15	+1.35 kB +435 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	507.76 123.44	507.76 123.44	+0 B +0 B
WebGPU	747.09 201.45	748.43 201.86	+1.33 kB +412 B
WebGPU Nodes	697.44 189.04	698.77 189.44	+1.33 kB +403 B

[ShiKirill]

@Mugen87

[sunag]

@ShiKirill Thanks for the PR, it might take me a bit longer to do the analysis; there are some things in the conditional flow which I was working on in another PR that I’d like to resolve before merge this.