Function generate_s4_r_wrapper 

pub fn generate_s4_r_wrapper(parsed_impl: &ParsedImpl) -> String

Generates the complete R wrapper string for an S4-style class.

Produces the following R code:

• Class definition: methods::setClass("<class>", slots = c(ptr = "externalptr")) with a single ptr slot holding the ExternalPtr to the Rust struct
• Constructor function: ClassName(...) that calls the Rust new constructor and wraps the result with methods::new("<class>", ptr = .val)
• S4 generics: methods::setGeneric(...) for each instance method (idempotent, always emitted rather than using conditional isGeneric() checks)
• S4 methods: methods::setMethod("<generic>", "<class>", function(x, ...) ...) dispatching to the Rust .Call() wrapper, extracting the ptr via x@ptr
• Static methods: regular functions named <class>_<method>(...)

Roxygen2 @exportMethod, @importFrom methods, and @slot tags are generated as appropriate.