Shadowing function arguments compiled sucessfully but generates invalid wgsl

```rust
#[wgsl]
pub mod my_shader {
pub fn my_func(x: f32) {
let x = 0.0;
}
}
```

compiles successfully, but when you try and run the wgsl:

```
Caused by:
In Device::create_shader_module, label = 'compute'

Shader 'compute' parsing error: redefinition of `x`
┌─ wgsl:68:12
│
68 │ fn my_func(x: f32) {
│ ^ previous definition of `x`
69 │ let x = 0.0;
│ ^ redefinition of `x`
```

Start with the provided Rust reproduction using the #[wgsl] entry point and my_func, then inspect the generated WGSL and reproduce the error when creating the shader module. Trace how function arguments and local bindings are emitted; done means the example no longer produces invalid WGSL and the generated shader is accepted.

Written by the indexing model from the issue text.