Function translate_shader 

pub fn translate_shader(hlsl: &str) -> Result<String>

Translate HLSL shader code to WGSL.

The pipeline is intentionally linear:

1. strip the MD2 shader_body { ... } outer wrapper,
2. type-substitute (float4 → vec4<f32>, …),
3. function-substitute (lerp → mix, tex2D → textureSample, …),
4. rewrite typed local declarations into WGSL var ident: T = …; form,
5. strip HLSL semantics (: POSITION, …).