Function generate_sidecar_accessors 

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

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

For each public #[r_data] field, generates:

• A getter FFI function (C_<crate>__mx_rdata_get_Type_field)

• A setter FFI function (C_<crate>__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 helpers of the S3 / S4 / env field methods (generate_field_syntax_helpers)

• With restored(refuse = "every_read"), the liveness check (C_<crate>__mx_rdata_live_Type, #1901) the field methods call

The generated statics are:

• __MX_CALL_DEF_RDATA_*: the R_CallMethodDef entries
• R_WRAPPERS_RDATA_{Type}: the R6 / S7 integration, when present
• R_SIDECAR_ACCESSORS_{Type}: the standalone accessors, exported and internal, which the wrapper writer picks between by the class’s field syntax (registry::SidecarAccessorsEntry, #1900). Only the internal rendering includes the field-syntax helpers.

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