G14 -- Rehearse every irreversible repository operation

A history rewrite, a purge, a force-push or a mass move is rehearsed on a scratch copy first and proven pair by pair with POSITIVE assertions (every purged path absent AND every kept file identical AND the pruned set equal to the derived set), never with absences alone. provenance: BFM R76; trial rewrite #1's empty-blob strip-list defect