fix(benchmark): harden paired utility result and evidence contracts

[tcconnally]

Summary

• Follow-up hardening for the paired coding-agent utility protocol delivered in bench(agent-utility): add a paired coding-agent memory lane with delivery, use, and outcome evidence #992 / PR feat(benchmark): add paired coding-agent utility protocol (#992) #993.
• Rejects duplicate arm roles, unbound case identities, contradictory delivery/mutation receipts, and unknown nested contract fields.
• Recomputes paired deltas and report projections from accepted observations.
• Verifies public evidence is actually sanitized before accepting its digest seal.

Verification

• Ruff clean.
• Focused utility suite: 22 passed.
• Full suite: 2557 passed, 8 skipped.

Related to #992.