feature: research ZK-proof based identity verification

[nanaf6203-bit]

Summary

KYC currently requires a centralized identity provider. Research a ZK-proof based alternative (Semaphore or Polygon ID) to reduce data exposure and improve user privacy.

Affected area

• src/lib/kyc.ts
• src/types/kyc.ts
• src/components/kyc/

Acceptance criteria

• Author an RFC in docs/ comparing Semaphore and Polygon ID for PropChain KYC flows.
• Identify integration points (off-chain issuer, on-chain verifier, recovery flow).
• Spike a minimal end-to-end proof in a sandbox environment.
• Document regulator compliance considerations (GDPR, financial KYC AML).

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Filed automatically as part of the PropChain code-review sweep.