Function str_iter_to_strsxp 

pub(crate) fn str_iter_to_strsxp<'a>(
    iter: impl ExactSizeIterator<Item = &'a str>,
) -> SEXP

Helper: allocate STRSXP and fill from a string iterator (checked).

Routes the alloc + protect through StrVecBuilder over a local ProtectScope; the CHARSXP policy (empty-string → R_BlankString) stays here via str_to_charsxp.