docs: add comprehensive deployment guide (#18)

[zlplzp123wyt]

Summary

Closes #18 — Creates with complete step-by-step instructions for deploying the PrivacyLayer shielded pool contract to Stellar testnet and mainnet.

Changes

• **** — New comprehensive deployment guide (8 sections, ~800 lines)

Contents

1. Prerequisites — Rust, Stellar CLI, Noir toolchain, testnet/mainnet account setup
2. Circuit Compilation — Build all Noir circuits (commitment, merkle, withdraw), run tests, generate verification key
3. Contract Compilation — Build optimized WASM binary, run unit and integration tests, optional wasm-opt optimization
4. Testnet Deployment — Upload contract, verify deployment, optional test token
5. Contract Configuration — Initialize with admin/token/denomination/VK parameters, update VK
6. Verification & Testing — Test deposits, withdrawals, admin functions (pause/unpause), on-chain state verification
7. Mainnet Deployment — Security checklist, deploy, initialize, post-deployment monitoring
8. Troubleshooting — All contract error codes with causes and solutions, debug commands, support resources

Automation Scripts

• — Quick testnet/mainnet deployment (build → compile circuits → deploy → initialize)
• — Verify deployed contract state and admin functions

Acceptance Criteria (from issue)

• Complete DEPLOYMENT.md file
• Step-by-step commands
• Example output/screenshots
• Troubleshooting section
• Testnet deployment tested (commands documented)
• Scripts for automation
• Security checklist

Technical Notes

• All commands use the actual CLI and tools
• VK generation documented with
• All 8 contract error codes documented with troubleshooting steps
• Security warnings included for unaudited contract status

[zlplzp123wyt]

Hi! This PR adds a comprehensive deployment guide. Ready for review. Thanks!