Function build_response 

pub fn build_response(session_flags: u16, blob: &[u8]) -> Vec<u8> 

Build a SESSION_SETUP response body (§2.2.5.2).

session_flags: 0x01 guest, 0x02 null session. The security buffer directly follows the 8-byte fixed body, i.e. offset 64+8=72 from the frame start.