Function generate_erased_wrapper 

fn generate_erased_wrapper(input: &DeriveInput) -> TokenStream

Generate the type-erased wrapper infrastructure for trait ABI dispatch.

For #[derive(ExternalPtr)] struct MyCounter { ... } generates:

• __MX_TAG_MyCounter - concrete type tag (FNV-1a hash of module_path + type)
• __MX_BASE_VTABLE_MyCounter - base vtable with trait/sidecar query
• __mx_wrap_MyCounter - constructor returning *mut mx_erased