feat(game-server): scaffold dedicated server toward player join

R&D branch for an authoritative Unreal game server (Prospect.Unreal).
The connection handshake already reaches PlayerController spawn; this
commit pushes it toward real players joining:

- Program.cs: target the Cycle station map + game mode (configurable via
  PROSPECT_MAP/GAMEMODE/PORT), proper host loop, and initialise a
  GameSession so NMT_Join -> Login no longer fails ("GameSession is null").
- UWorld.WelcomePlayer: send the world's real map/game mode to the client
  instead of the UE ThirdPerson template.
- Dockerfile.gameserver + .gitea/workflows/game-server.yml: build/push a
  separate image (the-cycle-game) on the game-server branch, ntfy notify.
- GAMESERVER.md: honest state + roadmap (player spawn + movement
  replication need live client testing; AI/loot intentionally deferred).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile.gameserver

@@ -0,0 +1,14 @@
+# The Cycle: Frontier — serveur de jeu dédié (EXPÉRIMENTAL / R&D).
+# Réimplémentation du serveur autoritaire Unreal (Prospect.Unreal). Build depuis les sources.
+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
+WORKDIR /src
+COPY src/ ./src/
+RUN dotnet publish src/Prospect.Server.Game/Prospect.Server.Game.csproj -c Release -o /app
+
+FROM mcr.microsoft.com/dotnet/runtime:8.0
+WORKDIR /app
+COPY --from=build /app ./
+# Le réseau Unreal est en UDP.
+EXPOSE 7777/udp
+# Configurable : PROSPECT_MAP / PROSPECT_GAMEMODE / PROSPECT_PORT
+ENTRYPOINT ["dotnet", "Prospect.Server.Game.dll"]