The unowned remainder is the four run-stderr assertions — msg content
for fail_msg/abort_msg, MANDATORY EMPTINESS for fail_nomsg/abort_bare
(rt_abort(NULL,0) writes nothing) — which the fixture protocol cannot
see (run exit only). The rows build the r957_* fixture sources in
place via the cstage driver; byte-id extends the lowering to wwstage
(the carrier's own argument). Owned legs cited, not duplicated: all
eight exit rows by the r957_assert_*/r957_abort_* fixtures, the
checker rejects by r957_assert_reject_*, byte-id by the
test-data-byteid blanket.
