Preserve GuardBench adapter extension evidence

[Evilander]

Summary

• preserve adapter-specific GuardBench result fields under adapterExtensions instead of dropping them during normalization
• allow optional JSON-shaped adapterExtensions in GuardBench raw and summary schemas
• add focused tests for passthrough, explicit extension fields, non-JSON rejection, and published row schema acceptance

Verification

• npm ci
• npm run build
• npx vitest run tests/guardbench-adapter-extensions.test.js tests/guardbench.test.js -t "GuardBench adapter extension evidence|external adapter result contract"
• npm run typecheck
• npm run bench:guard:adapter-smoke
• node benchmarks/validate-guardbench-artifacts.mjs --dir benchmarks/output/adapter-smoke