feat: add TicTacToe example project

Implement a complete Tic-Tac-Toe game using OECS to demonstrate
ECS patterns, including:
- Command pattern for placing marks
- System groups for logical updates and rendering
- CSV loading for initial board state
- Reactivity logging for observing entity and component changes

examples/TicTacToe/Singletons/GameState.cs

@@ -0,0 +1,14 @@
+using MessagePack;
+
+namespace TicTacToe;
+
+/// <summary>
+/// Global game state stored on the singleton entity.
+/// </summary>
+[MessagePackObject]
+public struct GameState
+{
+    [Key(0)] public Player CurrentPlayer;
+    [Key(1)] public GameStatus Status;
+    [Key(2)] public int MoveCount;
+}

examples/TicTacToe/Singletons/GameStatus.cs

@@ -0,0 +1,9 @@
+namespace TicTacToe;
+
+public enum GameStatus : byte
+{
+    Playing = 0,
+    XWon = 1,
+    OWon = 2,
+    Draw = 3
+}