* feat(internal): add secure_tcp_socket for TLS/SSL support

Implemented SSL/TLS wrapper for ASIO TCP socket:
- Created secure_tcp_socket class wrapping asio::ssl::stream
- Added async_handshake() for SSL handshake (client/server mode)
- Implemented encrypted async_read_some() and async_write()
- Thread-safe callback registration with mutex protection
- Support for receive and error callbacks
- Start/stop read loop control

This provides the foundation for TLS/SSL encrypted communication
on top of existing TCP infrastructure.

* feat(server): add secure_messaging_server with TLS/SSL support

Implemented secure server for encrypted TCP communication:
- Created secure_session class for SSL/TLS encrypted sessions
- Implemented secure_messaging_server with SSL context management
- Loads SSL certificate and private key at initialization
- Performs SSL handshake before data transmission
- Inherits session cleanup and backpressure from messaging_server
- Supports all monitoring and metrics collection features

SSL Configuration:
- Uses TLS 1.2+ (no SSLv2)
- Loads certificate chain and private key from PEM files
- Server-side handshake for incoming connections
- Automatic session cleanup every 30 seconds

This provides production-ready TLS/SSL encrypted server functionality.

* feat(client): add secure_messaging_client with TLS/SSL support

Implemented secure client for encrypted TCP communication:
- Created secure_messaging_client class for SSL/TLS encrypted connections
- Performs SSL handshake after TCP connection establishment
- Supports certificate verification (optional, configurable)
- Client-side handshake with server certificate validation
- Uses default system certificate paths for verification
- Synchronous handshake with 10-second timeout

Features:
- Encrypted data transmission via send_packet()
- Automatic connection state management
- Thread-safe operations with atomic flags
- Graceful error handling and cleanup
- Compatible with secure_messaging_server

This completes the TLS/SSL client-server implementation.

* feat(build): Add BUILD_TLS_SUPPORT option for conditional SSL/TLS compilation

- Add BUILD_TLS_SUPPORT option (default ON)
- Move TLS/SSL sources to conditional compilation block
- Update WebSocket section to avoid duplicate OpenSSL finding
- Add TLS/SSL support to build configuration summary

* docs: Update documentation for TLS/SSL support implementation

- Mark Issue #4 (Add TLS/SSL Support) as completed in IMPROVEMENTS.md
- Add v1.4.0 release notes to CHANGELOG.md with detailed TLS/SSL features
- Update Version Support Matrix to include v1.4.0 as current version

* docs: Update all documentation for TLS/SSL support

- Update README.md and README_KO.md with TLS/SSL features and examples
- Add TLS/SSL to Core Features and Protocol Support sections
- Update architecture diagrams to include secure components
- Add TLS/SSL secure server and client examples
- Add OpenSSL to dependencies (required)
- Update IMPROVEMENTS_KO.md to mark Issue #4 as completed
- Update CHANGELOG_KO.md with v1.4.0 release notes (TLS/SSL support)
- Update Version Support Matrix to include v1.4.0 as current version

* refactor: remove unused includes from secure_session.cpp

- Remove unused <type_traits> header
- Remove unused send_coroutine.h include
- Build verified successfully