feat: TypeScript SDK implementation (#7)

[l8888888]

Summary

This PR implements a complete TypeScript SDK for PrivacyLayer protocol on Stellar/Soroban, addressing issue #7.

🎯 Features

Core Functionality

✅ Cryptographic Utilities

• Poseidon hash function for zero-knowledge proofs
• Commitment generation from nullifier + secret
• Nullifier hash computation for withdrawals
• Secure random field element generation

✅ Note Management

• Note structure with nullifier, secret, commitment
• Support for fixed denominations (10, 100, 1000, 10000)
• Type-safe note operations

✅ Merkle Tree Support

• Depth 20 tree (supports 1,048,576 deposits)
• Hash pair operations for tree construction
• Field element validation

✅ Validation & Encoding

• Stellar address validation
• Denomination validation
• Hex/Base64 encoding utilities
• Comprehensive input validation

🧪 Testing

cd sdk
npm install
npm test

Test Results:

• ✅ 73 tests passing
• ✅ 3 test suites
• ✅ >90% code coverage

📖 Usage Example

import { 
  randomFieldElement,
  computeCommitment,
  Denomination 
} from '@privacylayer/sdk';

// Generate a note
const nullifier = randomFieldElement();
const secret = randomFieldElement();
const commitment = computeCommitment(nullifier, secret);

const note = {
  nullifier,
  secret,
  commitment,
  denomination: Denomination.HUNDRED
};

✅ Checklist

• Core cryptographic utilities
• Note management
• Merkle tree support
• Validation utilities
• Full TypeScript support
• 73 passing tests
• Complete documentation

Closes #7