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-proto/proto/peer_rpc.proto

@@ -348,6 +348,7 @@ message PeerConnNoiseMsg1Pb {
   optional uint32 a_session_generation = 3;
   common.UUID a_conn_id = 4;
   string client_encryption_algorithm = 5;
+  repeated string features = 6;
 }
 
 message PeerConnNoiseMsg2Pb {
@@ -361,6 +362,7 @@ message PeerConnNoiseMsg2Pb {
   common.UUID a_conn_id_echo = 8;
   optional bytes secret_proof_32 = 9;
   string server_encryption_algorithm = 10;
+  repeated string features = 11;
 }
 
 message RelayNoiseMsg1Pb {