A bodied fn with a bare C-style `...` was silently accepted by cstage
and SEGFAULTED wwstage (resolvefnbody walked a typeless `...` param).
Gate it: bare C-`...` is allowed only on bodiless decls (extern /
@symbol prototypes), the real FFI path; Hare-style `T...` is unaffected.
ww restricts C-`...` to bodiless decls pending vastart/vaarg/vaend
builtins (#16); harec permits bodied C-variadic fns (check.c:3656) -- a
documented divergence, reopened when #16 lands.
Test 852 runs both stages; its reject rows require the gate's diagnostic
(not merely a nonzero exit), so a crash can't pass them vacuously.
Hojun-Cho
1f1efb273a