feat: initial etrelay UDP relay with master heartbeat

diagrams/player-session.puml

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+@startuml
+skinparam shadowing false
+title Player: one session per client address
+
+actor "Player\n1.2.3.4:5555" as player
+participant "etrelay\nfront :27960" as relay
+participant "session task\n(ephemeral socket)" as session
+participant "ET server" as et
+
+player -> relay : getchallenge / connect / game packets
+relay -> session : first packet: open session
+session -> et : forward (source = ephemeral port)
+et --> session : reply
+session --> player : reply sent from front:27960
+...packets both ways, deadline pushed back on each packet...
+...2 min without any packet...
+session -> session : close (socket dropped)
+note over relay : next packet from 1.2.3.4:5555\nopens a new session
+@enduml