Widens the completed-hook's real race window (normally instructions-wide, needing
~2x-core host load to hit by chance per #399) by injecting a bounded sleep into the
test clock's completion-stamp read. This makes the ordering invariant — a test must
wait for isCompletionStampedForTest, not just DONE, before advancing the clock past
the TTL — fail deterministically on the first run when the barrier is removed, and
pass deterministically with it present. No production code changed.
Dai Ha