Function server_handshake 

pub async fn server_handshake<R, W>(
    reader: &mut R,
    writer: &mut W,
    server_keypair: &Keypair,
    local_creds: &PeerCredentials,
    remote_creds: &PeerCredentials,
) -> Result<NoiseTransport>
where
    R: AsyncRead + Unpin,
    W: AsyncWrite + Unpin,

Perform the server-side (responder) Noise IK handshake.

Called after stream.into_split() and extract_ucred(), before the read/write loops. The server’s static keypair was generated at startup and the client’s identity is verified via the Noise handshake + UCred prologue binding.

IK handshake (responder perspective):

1. Read message 1 from initiator (contains initiator’s ephemeral + encrypted static)
2. Write message 2 to initiator (contains responder’s ephemeral)
3. Handshake complete — derive transport keys

Errors

Returns an error if the handshake fails or times out.