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>

src/Prospect.Server.Game/Program.cs

@@ -1,4 +1,5 @@
-using Prospect.Unreal.Core;
+using Prospect.Unreal.Core;
+using Prospect.Unreal.Net.Actors;
 using Prospect.Unreal.Runtime;
 using Serilog;
 
@@ -7,10 +8,10 @@ namespace Prospect.Server.Game;
 internal static class Program
 {
     private const float TickRate = (1000.0f / 60.0f) / 1000.0f;
-    
+
     private static readonly ILogger Logger = Log.ForContext(typeof(Program));
     private static readonly PeriodicTimer Tick = new PeriodicTimer(TimeSpan.FromSeconds(TickRate));
-    
+
     public static async Task Main()
     {
         Console.CancelKeyPress += (_, e) =>
@@ -18,37 +19,55 @@ internal static class Program
             Tick.Dispose();
             e.Cancel = true;
         };
-        
+
         Log.Logger = new LoggerConfiguration()
             .MinimumLevel.Verbose()
             .Enrich.FromLogContext()
             .WriteTo.Console(outputTemplate: "[{Timestamp:HH:mm:ss} {Level:u3}] ({SourceContext,-52}) {Message:lj}{NewLine}{Exception}")
             .CreateLogger();
-        
-        Logger.Information("Starting Prospect.Server.Game");
 
-        // Prospect:
-        //  Map:        /Game/Maps/MP/Station/Station_P
-        //  GameMode:   /Script/Prospect/YGameMode_Station
-        
-        var worldUrl = new FUrl
-        {
-            Map = "/Game/ThirdPersonCPP/Maps/ThirdPersonExampleMap"
-        };
-        
+        // The Cycle: Frontier authoritative game server.
+        // We start on the station map (the simplest shared space to get two players spawned
+        // and moving); the raid maps (Bright Sands, …) come once spawn + movement replication
+        // work end to end. AI and loot are intentionally out of scope for now.
+        var map = Environment.GetEnvironmentVariable("PROSPECT_MAP") ?? "/Game/Maps/MP/Station/Station_P";
+        var gameMode = Environment.GetEnvironmentVariable("PROSPECT_GAMEMODE") ?? "/Script/Prospect/YGameMode_Station";
+        var port = int.TryParse(Environment.GetEnvironmentVariable("PROSPECT_PORT"), out var p) ? p : 7777;
+
+        Logger.Information("Starting Prospect.Server.Game — map={Map} gameMode={GameMode} port={Port}", map, gameMode, port);
+
+        var worldUrl = new FUrl { Map = map, Port = port };
+        worldUrl.Options.Add($"game={gameMode}");
+
         await using (var world = new ProspectWorld())
         {
             world.SetGameInstance(new UGameInstance());
             world.SetGameMode(worldUrl);
+
+            // The game mode needs a GameSession, otherwise NMT_Join -> SpawnPlayActor -> Login
+            // fails with "GameSession is null" and the player is never spawned.
+            var authGameMode = world.GetAuthGameMode();
+            if (authGameMode != null && authGameMode.GameSession == null)
+            {
+                authGameMode.GameSession = new AGameSession();
+            }
+
             world.InitializeActorsForPlay(worldUrl, true);
-            world.Listen();
-        
+
+            if (!world.Listen())
+            {
+                Logger.Fatal("Failed to start listening (port {Port} already in use?)", port);
+                return;
+            }
+
+            Logger.Information("Server listening on :{Port}. Waiting for players…", port);
+
             while (await Tick.WaitForNextTickAsync())
             {
                 world.Tick(TickRate);
             }
         }
-        
+
         Logger.Information("Shutting down");
     }
-}
+}