Function generate_sidecar_accessors 

fn generate_sidecar_accessors(
    input: &DeriveInput,
    info: &SidecarInfo,
) -> Result<TokenStream>

Generate sidecar accessor constants and extern "C-unwind" functions.

For each public #[r_data] field, generates:

• A getter FFI function (C__mx_rdata_get_Type_field)
• A setter FFI function (C__mx_rdata_set_Type_field)
• R_CallMethodDef entries for routine registration
• R wrapper function code (roxygen-documented)
• Class-integration code for R6 / S7 (active bindings / properties)

The generated constants are:

• RDATA_CALL_DEFS_{TYPE}: slice of R_CallMethodDef for registration
• R_WRAPPERS_RDATA_{TYPE}: string literal of R wrapper code

Returns Err if the struct has generic type parameters (.Call entrypoints cannot be generic).