Source OnDataPathRekeyed from the WGWatcher's per-handshake callback
(onWGCheckSuccess), which fires only on a fresh handshake, and OnDataPathDown
from the handshake-timeout path. A fresh handshake clocks the next chained
KEM exchange pushed over the data-path UDP transport.
Learn the peer's data-path endpoint from the signalling offer/answer: its WG
overlay IP combined with the advertised pq UDP port (SetRemotePort -> AddPeer).
Registering here is safe before the tunnel is up because sends only ever fire
once it is (clocked by OnDataPathRekeyed). RemovePeer is wired at peer teardown
(engine.removePeer), not on transient disconnect.
riccardom