feat: Define contract-side path for private withdrawals

[fadee26]

Summary

Implement the contract-side path for private withdrawals compatible with the VeilLend privacy roadmap.

Changes Made

1. Added shielded deposit/withdraw functions to Soroban contract
2. Reserved interfaces for future ZK proof verification
3. Documented withdrawal flow assumptions for the privacy roadmap (Phase 1, 2, 3)
4. Maintained full backward compatibility with existing lending scaffold

Acceptance Criteria Met

✅ Withdrawal flow assumptions documented in code comments
✅ Contract interfaces for future proof verification reserved
✅ Design does not break current lending scaffold
✅ Compatible with GrantFox OSS campaign requirements

Privacy Roadmap Context

• Phase 1 (Current): Basic validation + privacy placeholders
• Phase 2 (Future): Integrate ZK proof verification
• Phase 3 (Future): Full privacy with ZK-SNARK/STARK

closes #34