Add corroboration/confidence signals for downstream synthesis guardrails

[VACInc]

Summary

• add structured per-result confidence/corroboration metadata (results[].confidence)
• add broker-level confidence rollup (payload.confidence) with weak-evidence and synthesis-heavy counts
• include provider/domain/evidence counts plus machine-readable weak-evidence reasons
• document the new metadata in README and add focused tests

Testing

• pnpm check
• pnpm test