Function generate_keypair_derand 

pub fn generate_keypair_derand(secret_key: &[u8; 32]) -> (SecretKey, PublicKey)

Generates a deterministic X-Wing keypair from the given 32-byte seed.

This is the deterministic variant of generate_keypair, intended for protocols that need reproducible key generation (e.g. HPKE’s DeriveKeyPair and test vectors). The same seed always yields the same keypair.