Same change as the reviewed FixedPatchStepper end-sync (cherry-picked
ad1d82c+512dcc9 above): the n steps are enqueued in order and the host
waits once; step(0) launches nothing; step_count advances after the wait
and keeps its entry value on failure. Launches, buffer roles and bits are
those of n step(1) calls. Mocked-driver host test covers the single
wait, launch/role equality with n step(1), step(0) and failure.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
