Replace the raw SHA-256 concat combiner with HKDF-SHA256 (crypto/hkdf, Go 1.24):
IKM = ML-KEM_ss || X25519_ss (draft-ietf-tls-ecdhe-mlkem order), salt = the
domain-separation label, info = full transcript (offer || answer) || canonicalised
peer identities. Keeps the transcript + identity binding while using a proper KDF.