Closes #428

Summary

Added comprehensive security module documentation (docs/SECURITY_GUIDE.md) covering all 6 security components:

• Audit Logger (audit_logger.h): HMAC-signed tamper-evident logging with JSON format, ISO 8601 timestamps, and OpenSSL 3.x/1.1.x compatibility
• Log Sanitizer (log_sanitizer.h): PII/sensitive data detection and masking (credit cards, SSNs, API keys, passwords, emails, IPs, phone numbers) with custom pattern support
• Path Validator (path_validator.h): Path traversal prevention, symlink validation, and filename character restrictions
• Secure Key Storage (secure_key_storage.h): Secure key generation (RAND_bytes), RAII memory management, and 0600 permission enforcement
• Signal Manager (signal_manager.h, signal_manager_interface.h): Signal-safe emergency log flushing for SIGSEGV/SIGABRT/SIGTERM/SIGINT

Documentation Content

• Threat model: Attacks defended against and security assumptions
• Architecture: Component integration diagrams and dependency graph
• API documentation: All public methods with parameters, return values, and error codes
• Usage examples: Minimal setup, high-security setup, and custom security layers
• Compliance considerations: SOX, HIPAA, GDPR, PCI-DSS requirements and implementation guidance
• FAQ: Common questions about OpenSSL fallback, key rotation, performance, and signal handling

Test Plan

• All 6 security headers documented with accurate API signatures
• Threat model and security architecture explained
• Each component's API fully documented
• Configuration examples for each component
• Integration with core logging explained
• Compliance considerations noted
• Document reviewed for technical accuracy
• Code examples verified against actual header files