[ZK] ZK reputation proofs

[Gbangbolaoluwagbemiga]

User story

A freelancer wants to prove "I have completed ≥ N jobs" or "my average rating ≥ X" to a
prospective client without revealing which clients, projects, or amounts are behind that record.

Background

Reputation is tracked on-chain (ratings.rs: get_average_rating, get_completed_escrows,
get_badge). Today these expose the full history. ZK lets the freelancer prove a threshold while
keeping specifics private.

What needs to be done

• Circuit: input = the freelancer's private rating/completion records (+ Merkle membership in
  the on-chain reputation set); public = threshold N / X and a commitment root
• Verifier per the architecture chosen in [ZK] Research spike: ZK verification on Soroban #35 (on-chain or attested)
• Contract entrypoint: verify_reputation_proof(...) returning pass/fail, with a nullifier to
  prevent replay
• Frontend: generate the proof and show a "verified reputation" badge

Acceptance criteria

• A valid proof for a true statement verifies; a false statement fails
• No private record is revealed on-chain
• Replay-protected

Relevant files

contracts/secureflow/src/ratings.rs, src/components/reputation-score.tsx

Depends on #35.