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/Commands/PlaceMarkCommand.cs

@@ -0,0 +1,50 @@
+using MessagePack;
+using OECS;
+
+namespace TicTacToe;
+
+/// <summary>
+/// Places the current player's mark on the cell at (Row, Col).
+/// Validates that the cell is empty and the game is still playing.
+/// </summary>
+[MessagePackObject]
+public struct PlaceMarkCommand : ICommand
+{
+    [Key(0)] public int Row;
+    [Key(1)] public int Col;
+
+    public void Execute(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        if (state.Status != GameStatus.Playing)
+            return;
+
+        // Copy to locals so the lambda in ForEach can capture them
+        // (this is a struct, so `this` cannot be captured directly).
+        int row = Row;
+        int col = Col;
+
+        // Find the cell entity at (Row, Col) that has no Mark.
+        var query = world.Query().With<Cell>().Without<Mark>().Build();
+        Entity? target = null;
+
+        world.ForEach(query, (Entity entity, ref Cell cell) =>
+        {
+            if (cell.Row == row && cell.Col == col)
+                target = entity;
+        });
+
+        if (target == null)
+            return; // Cell already occupied or invalid position.
+
+        // Place the mark.
+        world.AddComponent(target.Value, new Mark { Player = state.CurrentPlayer });
+        world.MarkModified<Mark>(target.Value);
+
+        // Advance turn.
+        state.MoveCount++;
+        state.CurrentPlayer = state.CurrentPlayer == Player.X ? Player.O : Player.X;
+        world.MarkModified<GameState>(World.SingletonEntity);
+    }
+}