feat(engine): default-to-fail posture + non-mutating verification run (U2,U3)
Behavioral/bug assertions now default to fail unless a bounded verification run
confirms them; static assertions keep the existing read-only judging path. Adds
mission-verification.ts (injected capability): disposable git checkout at a
trusted revision, explicit isolating sandbox backend with fail-closed + scrubbed
env, fixed command template + validated test path, merge-base pre-fix baseline
(rejects pass-on-both), git-clean post-condition, inconclusive as a first-class
verdict. Judge session stays tools:readonly.
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