Function negotiate 

pub fn negotiate(mine: &PeerProfile, theirs: &PeerProfile) -> Negotiated

Negotiate how to send TO a peer from my profile + the peer’s advertised one. CONSERVATIVE: a capability is used only when BOTH sides expose it. This is where “expose properly” pays off — the sender automatically restricts itself to exactly the surface the receiver published, so it never ships a form the receiver can’t decode, won’t run code the receiver declined, and stays under the receiver’s size budget.