add etrelay UDP relay + etmasters script + update README

relay/diagrams/architecture.puml

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+@startuml
+skinparam shadowing false
+title After: everything goes through the front
+
+actor Player as player
+node "Master\netmaster.net:27950" as master
+
+node "Front (public IP)" {
+  component "etrelay\n0.0.0.0:27960" as relay
+}
+
+node "Home" {
+  component "nginx stream\n<wg-peer>:27960" as nginx
+  component "ET server\n<et-server>:27960" as et
+}
+
+player <--> relay : game traffic
+master <--> relay : heartbeat / getChallenge / getInfo
+relay <--> nginx : WireGuard (wg0)
+nginx <--> et : LAN
+@enduml

relay/diagrams/before.puml

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+@startuml
+skinparam shadowing false
+title Before: heartbeat leaves through the home box
+
+node "Master\netmaster.net:27950" as master
+node "Home box\nhome IP" as box
+node "Front\npublic IP" as front
+node "ET server\n<et-server>:27960" as et
+
+et --> box : heartbeat
+box --> master : heartbeat (source = home IP)
+master --> box : getChallenge\n(to home IP: dropped)
+front ..> et : players (DNAT + wg + nginx stream)
+@enduml

relay/diagrams/heartbeat-states.puml

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+@startuml
+skinparam shadowing false
+title Heartbeat decision (every 5 min)
+
+[*] --> Down
+Down --> Up : probe answers\n/ heartbeat EnemyTerritory-1
+Up --> Up : probe answers\n/ heartbeat EnemyTerritory-1
+Up --> Down : no answer\n/ heartbeat ETFlatline-1 (once)
+Down --> Down : no answer\n/ nothing sent
+@enduml

relay/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

relay/diagrams/registration.puml

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+@startuml
+skinparam shadowing false
+title Master registration (every 5 min)
+
+participant "Master\netmaster.net:27950" as master
+participant "etrelay\nfront :27960" as relay
+participant "ET server\n(via wg + nginx)" as et
+
+== Liveness probe (dedicated ephemeral socket) ==
+relay -> et : getinfo etrelay
+et --> relay : infoResponse
+note right of relay : up: send heartbeat\nno answer x3: skip (or flatline once)
+
+== Heartbeat (public socket) ==
+relay -> master : heartbeat EnemyTerritory-1\nsource = front:27960
+
+== Master checks: handled like any client ==
+master -> relay : getChallenge\nto front:27960
+relay -> relay : new session for master:27950
+relay -> et : getChallenge
+et --> relay : challengeResponse
+relay --> master : challengeResponse\nsource = front:27960
+master -> relay : getInfo
+relay -> et : getInfo
+et --> relay : infoResponse (hostname, map, players)
+relay --> master : infoResponse\nsource = front:27960
+note left of master : lists <front-ip>:27960
+@enduml

relay/diagrams/tasks.puml

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+@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