Add confidential memo hash registry

[edehvictor]

Summary

Add a registry for hashed payroll memo references so contracts can track memo integrity without storing memo content.

Why This Matters

Payroll memos may contain sensitive operational context, so the contract should only preserve verifiable commitments.

Suggested Files

• contracts/src/memos.rs
• contracts/src/storage.rs
• contracts/src/events.rs
• contracts/tests/memo_hash_registry.rs

Tasks

• Store memo hashes by employer, period, and batch identifier.
• Reject empty or duplicate memo hashes for the same payroll run.
• Emit privacy-safe memo registered events.
• Add tests for duplicate, missing, and valid memo hashes.

Acceptance Criteria

• Memo content is never stored on-chain.
• Duplicate memo hashes are rejected when scoped to the same run.
• Events expose only safe identifiers and hashes.
• Tests cover successful and invalid registration.

Coordination

Join the Telegram channel for proper coordination: https://t.me/zkpayroll