Add dest_coords to SessionAck for return-path routing

SessionAck previously only carried the responder's coordinates
(src_coords). When the return path diverged from the forward path
(e.g., after tree reconvergence), transit nodes on the return path
lacked the initiator's coordinates and couldn't route the SessionAck
back, causing handshake timeouts.

Add dest_coords (initiator's coordinates) to the SessionAck wire
format, mirroring SessionSetup's design. Transit nodes now cache both
endpoints' coordinates when forwarding a SessionAck, making the return
path self-sufficient regardless of path asymmetry.

Root cause confirmed by churn-20 sim log analysis: the n04-n14
handshake failure was caused by n15 (return-path transit) lacking
n04's coordinates, not by stale tree routes through a downed node.

src/node/handlers/forwarding.rs

@@ -138,8 +138,14 @@ impl Node {
                             ack.src_coords,
                             now_ms,
                         );
+                        self.coord_cache_mut().insert(
+                            datagram.dest_addr,
+                            ack.dest_coords,
+                            now_ms,
+                        );
                         debug!(
                             src = %datagram.src_addr,
+                            dest = %datagram.dest_addr,
                             "Cached coords from SessionAck"
                         );
                     }

src/node/handlers/session.rs

@@ -364,9 +364,9 @@ impl Node {
             }
         };
 
-        // Build and send SessionAck
+        // Build and send SessionAck (include initiator's coords for return-path warming)
         let our_coords = self.tree_state.my_coords().clone();
-        let ack = SessionAck::new(our_coords).with_handshake(msg2);
+        let ack = SessionAck::new(our_coords, setup.src_coords).with_handshake(msg2);
         let ack_payload = ack.encode();
         let my_addr = *self.node_addr();
         let mut datagram = SessionDatagram::new(my_addr, *src_addr, ack_payload.clone())