Require substantive independent exact-tree review before merging

Treat empty or evidence-free audit output as unusable. Retry that reviewer once with an explicit evidence-or-blocker contract; if the second result is also empty, replace only the failed reviewer with a fresh, narrower independent reviewer while allowing other valid reviews to continue. Keep the exact tree frozen and the merge gate closed until substantive, evidence-backed review results are obtained. After infrastructure restarts, resume existing reviewers with their prior context rather than duplicating their work.