Function permanent_charsxp 

#[doc(hidden)]
pub(crate) fn permanent_charsxp(name: &CStr) -> SEXP

Get a permanent CHARSXP for a string by going through Rf_install + PRINTNAME.

Symbols are never GC’d, so the CHARSXP from PRINTNAME is valid forever. This avoids the Rf_mkCharLenCE hash lookup on repeated calls.

Used by cached_strsxp! — pub(crate) so the macro can reference it from any module.