Hybrid encapsulate — performs X25519 ECDH + ML-KEM encapsulation, then derives a combined shared secret via HKDF.
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.