Function createEncapsulationKeypair

A fresh encapsulation key pair for scheme (X25519 by default). Without rng every scheme draws from the secure generator (keypair). With rng (keypair_using) the ML-KEM schemes throw General before drawing anything, as the reference does: their key generation takes no caller-supplied generator. Use MLKEMPrivateKey.keypair(level, { rng }) or EncapsulationPrivateKey.mlkemKeypair(level, { rng }) for a seeded ML-KEM pair.