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/Components/Cell.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+
+namespace TicTacToe;
+
+/// <summary>
+/// Identifies a board position. One entity per cell.
+/// </summary>
+[MessagePackObject]
+public struct Cell
+{
+    [Key(0)] public int Row;
+    [Key(1)] public int Col;
+}

examples/TicTacToe/Components/Mark.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+
+namespace TicTacToe;
+
+/// <summary>
+/// A mark placed on a cell by a player.
+/// Only present on cells that have been claimed.
+/// </summary>
+[MessagePackObject]
+public struct Mark
+{
+    [Key(0)] public Player Player;
+}

examples/TicTacToe/Components/Player.cs

@@ -0,0 +1,8 @@
+namespace TicTacToe;
+
+public enum Player : byte
+{
+    None = 0,
+    X = 1,
+    O = 2
+}