Implement PQC Handshake and Data Encryption

Implement the initial secure channel establishment between two nodes using PQC. Use a Post-Quantum Key Encapsulation Mechanism (KEM) like Kyber and a Post-Quantum Digital Signature Algorithm (DSA) like Dilithium (e.g., via `pqc-py` or a similar library) to securely exchange a classical AES-256 key. Demonstrate its use to encrypt and decrypt a sample message.