Hybrid encapsulate — performs X25519 ECDH + ML-KEM encapsulation, then derives a combined shared secret via HKDF.
ML-KEM security level (512, 768, or 1024).
Recipient's X25519 public key (hex).
Recipient's ML-KEM public key (hex).
Hybrid encapsulate — performs X25519 ECDH + ML-KEM encapsulation, then derives a combined shared secret via HKDF.