Function expand_struct 

fn expand_struct(
    attr: TokenStream,
    item: TokenStream,
    item_struct: &ItemStruct,
) -> TokenStream

Dispatches #[miniextendr] on a struct to the correct derive path.

Decision logic:

• ALTREP attrs (class/base): compile error with migration guidance (ALTREP via #[miniextendr] is removed — use the #[derive(Altrep*)] family)
• list mode: #[derive(IntoList, TryFromList, PreferList)]
• dataframe mode: #[derive(DataFrameRow)] (the companion gets IntoR)
• Default or explicit externalptr: ExternalPtr
• prefer = "native": ExternalPtr (without the IntoExternalPtr marker, #1283) + PreferRNativeType marker