Function generate_s3_r_wrapper 

pub fn generate_s3_r_wrapper(parsed_impl: &ParsedImpl) -> String

Generates the complete R wrapper string for an S3-style class.

Produces the following R code:

• Constructor: new_<class>(...) function that calls the Rust new constructor and wraps the result with structure(.val, class = "<class>")
• S3 generics: for each instance method, a UseMethod() generic is created (unless overriding an existing generic via #[miniextendr(generic = "...")])
• S3 methods: <generic>.<class> functions dispatching to the Rust .Call() wrapper, with the ExternalPtr extracted from x
• Static methods: regular functions named <class>_<method>(...)
• Class environment: ClassName <- new.env(parent = emptyenv()) for Class$new() syntax and trait namespace compatibility

Custom double-dispatch patterns (e.g., vec_ptype2.a.b) are supported via #[miniextendr(generic = "...", class = "...")] attributes.