feat(peer): echo liveness probes on data traffic (#2497)

* feat(peer): echo liveness probes on data traffic

Advertise a liveness-echo capability during classic and Noise
handshakes. After a ping failure, tag outgoing peer packets with a
short probe token and accept only the matching echoed token as
round-trip proof.

Keep one ping request outstanding and coalesce scheduler triggers so
high traffic cannot reorder timeout results. Preserve one-way failure
detection because unrelated ingress never clears the loss counter.

* test(three_node): relax disconnect wait for sequential pingpong

proxy_three_node_disconnect_test assumed the old pingpong timing,
where overlapping pings failed fast and the connection closed well
inside the 11s wait (see the old [4, 9)s comment).

The liveness-echo change keeps one ping outstanding: each failure
now takes a full 2s timeout, so the fifth consecutive failure and
the connection close land at ~11s. Both proto variants timed out at
the 11s bound in CI. Widen the wait to 15s and update the timing
comment.

easytier/src/tests/three_node.rs

@@ -1541,9 +1541,9 @@ pub async fn proxy_three_node_disconnect_test(#[values("tcp", "wg")] proto: &str
                         .any(|r| *r == inst4.peer_id())
                 },
                 // 0 down, assume last packet is recv in -0.01
-                // [2, 7) send ping
-                // [4, 9) ping fail and close connection
-                Duration::from_secs(11),
+                // one ping outstanding at a time, each waits up to 2s:
+                // 5 consecutive failures close the connection at ~[4, 11)
+                Duration::from_secs(15),
             )
             .await;