Function choices_entry_tokens 

pub(crate) fn choices_entry_tokens(
    cfg_attrs: &[Attribute],
    entry_ident: &Ident,
    placeholder: &str,
    choices_ty: &Type,
    preferred_default: &str,
) -> TokenStream

Emit the MX_MATCH_ARG_CHOICES static + its linkme registration.

Factored so lib.rs (standalone fns) and miniextendr_impl.rs (impl methods) can’t drift apart — both previously open-coded the same quote! block.

preferred_default is the unquoted form of the user’s default = "..." (e.g. "zstd"). Pass "" when the user supplied no default — the write pass then keeps the natural enum order.