Store one current verdict per requirement (#22)

* spec: define stable verdict file contract

* feat: supersede verdicts by requirement

* test: expect stable verdict path in audits

* test: strengthen verdict migration boundaries

* fix: address verdict review findings

* test: harden reviewer instruction boundaries

* test: allow CI time for prune matrix

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Co-authored-by: Panopticon Agent <panopticon-agent@users.noreply.github.com>

Author: tylerwillis