isPlainFreeFn / isPlainFreeFnDecl excluded a #foreign body but classified
an empty-block extern fn as a plain free function, so existing extern fns
were wrongly counted in the bare-call ambiguity verdict (and eligible for
the out-of-line-slot / shadow-author pass). Both predicates now also
exclude extern_export == .extern_ (an external C symbol with no
sx-lowerable body, name-keyed first-wins dispatch like #foreign); export
keeps a real body and stays plain-free. Greens example 1230 — same-name
extern authors compile like their #foreign twins (0729).
646 corpus / 444 unit, 0 failed.