feat: implement security features (HMAC signing, replay protection)

Security Features:
• HMAC-SHA256 message signing for integrity verification
• Signature verification with constant-time comparison
• Replay protection (timestamp freshness + nonce deduplication)
• SecurityManager class for signing and verification
• KeyManager class for key storage and retrieval

Technical Implementation:
• Deterministic canonical string creation for signing
• Tamper detection for any message modification
• Configurable timestamp validity window (default 5 minutes)
• 60-second clock skew tolerance
• Secure random key generation (32 bytes)

Tests:
• 40+ security tests covering all features
• Signing and verification roundtrips
• Tamper detection tests
• Replay attack simulations
• Key management tests
• Integration tests with binary encoding

Example:
• 06_security_features.py with 8 demonstrations
• Performance benchmarks (~1-2ms overhead)
• Best practices and production checklist

Performance:
• 1000 signatures: ~1-2ms per signature
• 1000 verifications: ~1-2ms per verification
• Minimal overhead for real-time applications

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Sergey Klein