feat: implement CardWars game engine

Implement the core CardWars game logic using an ECS-based architecture.
This includes:
- Card effect registry and various card effect implementations.
- Game phase management (Setup, Play, Flip, Scoring, Cleanup).
- Command system for player actions (PlayCard, FlipCard, etc.).
- Component-based game state and entity relationships.
- Automated game setup and scoring systems.
- Unit tests for game setup, card effects, and play logs.

Game.CardWars/Singletons/GameState.cs

@@ -0,0 +1,18 @@
+using MessagePack;
+
+namespace Game.CardWars;
+
+/// <summary>
+/// Global game state on the singleton entity.
+/// </summary>
+[MessagePackObject]
+public record struct GameState
+{
+    [Key(0)] public GamePhase Phase;
+    [Key(1)] public int PlayerCount;
+    [Key(2)] public int CurrentPlayerIndex;
+    [Key(3)] public int StartingPlayerIndex;
+    [Key(4)] public int RoundNumber;
+    [Key(5)] public uint Seed;
+    [Key(6)] public bool PlayPhaseEnded;
+}

Game.CardWars/Singletons/PlayerScores.cs

@@ -0,0 +1,14 @@
+using MessagePack;
+
+namespace Game.CardWars;
+
+/// <summary>
+/// Per-player score tracking.
+/// </summary>
+[MessagePackObject]
+public record struct PlayerScores
+{
+    [Key(0)] public int[] Scores;
+    [Key(1)] public CastleColor[] WonCastles;
+    [Key(2)] public int Count;
+}