feat: initial etrelay UDP relay with master heartbeat

diagrams/tasks.puml

@@ -0,0 +1,26 @@
+@startuml
+skinparam shadowing false
+title Tasks and data flow
+
+rectangle "relay loop (main task)" as loop {
+  card "public socket\nrecv_from" as recv
+  card "sessions:\nHashMap<client, Sender>" as map
+}
+
+rectangle "session task (one per client)" as st {
+  card "mpsc Receiver\n(queue: 64)" as rx
+  card "upstream socket\n(connected to ET)" as up
+}
+
+rectangle "heartbeat task" as hb {
+  card "interval 5 min" as tick
+  card "probe socket" as probe
+}
+
+recv --> map : lookup client
+map --> rx : try_send(packet)\n(full: drop)
+rx --> up : send
+up --> recv : recv, then public.send_to(client)
+tick --> probe : getinfo
+tick --> recv : heartbeat via public socket
+@enduml