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/Systems/RenderSystem.cs

@@ -0,0 +1,67 @@
+using OECS;
+
+namespace TicTacToe;
+
+/// <summary>
+/// Prints the board to the console.
+/// </summary>
+public class RenderSystem : ISystem
+{
+    public QueryDescriptor Query { get; }
+
+    public RenderSystem(World world)
+    {
+        Query = world.Query().With<Cell>().Build();
+    }
+
+    public void Run(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        // Build a 3×3 grid.
+        var grid = new char[3, 3];
+        for (int r = 0; r < 3; r++)
+            for (int c = 0; c < 3; c++)
+                grid[r, c] = '.';
+
+        // Fill in placed marks.
+        var markQuery = world.Query().With<Cell>().With<Mark>().Build();
+        world.ForEach(markQuery, (Entity entity, ref Cell cell, ref Mark mark) =>
+        {
+            grid[cell.Row, cell.Col] = mark.Player == Player.X ? 'X' : 'O';
+        });
+
+        Console.WriteLine();
+        Console.WriteLine("  Tic-Tac-Toe");
+        Console.WriteLine("  ═══════════");
+        Console.WriteLine();
+        Console.WriteLine("    0   1   2");
+        Console.WriteLine("  ┌───┬───┬───┐");
+        for (int r = 0; r < 3; r++)
+        {
+            Console.Write($"{r} │");
+            for (int c = 0; c < 3; c++)
+            {
+                Console.Write($" {grid[r, c]} ");
+                if (c < 2) Console.Write("│");
+            }
+            Console.WriteLine("│");
+            if (r < 2) Console.WriteLine("  ├───┼───┼───┤");
+        }
+        Console.WriteLine("  └───┴───┴───┘");
+        Console.WriteLine();
+
+        // Status line.
+        var statusText = state.Status switch
+        {
+            GameStatus.Playing => $"{(state.CurrentPlayer == Player.X ? 'X' : 'O')}'s turn (move {state.MoveCount + 1})",
+            GameStatus.XWon => "X wins!",
+            GameStatus.OWon => "O wins!",
+            GameStatus.Draw => "It's a draw!",
+            _ => ""
+        };
+        Console.WriteLine($"  {statusText}");
+        Console.WriteLine();
+        Console.Write("  Enter row col (e.g. \"1 2\"): ");
+    }
+}

examples/TicTacToe/Systems/WinCheckSystem.cs

@@ -0,0 +1,89 @@
+using OECS;
+
+namespace TicTacToe;
+
+/// <summary>
+/// After each move, checks whether the game has been won or drawn.
+/// Updates the GameState singleton accordingly.
+/// </summary>
+public class WinCheckSystem : ISystem
+{
+    public QueryDescriptor Query { get; }
+
+    public WinCheckSystem(World world)
+    {
+        Query = world.Query().With<Cell>().With<Mark>().Build();
+    }
+
+    public void Run(World world)
+    {
+        ref var state = ref world.GetSingleton<GameState>();
+
+        if (state.Status != GameStatus.Playing)
+            return;
+
+        // Build a 3×3 grid of marks.
+        var grid = new Player[3, 3];
+
+        world.ForEach(Query, (Entity entity, ref Cell cell, ref Mark mark) =>
+        {
+            grid[cell.Row, cell.Col] = mark.Player;
+        });
+
+        // Check rows.
+        for (int r = 0; r < 3; r++)
+        {
+            if (TryGetWinner(grid[r, 0], grid[r, 1], grid[r, 2], out var winner))
+            {
+                SetWinner(world, ref state, winner);
+                return;
+            }
+        }
+
+        // Check columns.
+        for (int c = 0; c < 3; c++)
+        {
+            if (TryGetWinner(grid[0, c], grid[1, c], grid[2, c], out var winner))
+            {
+                SetWinner(world, ref state, winner);
+                return;
+            }
+        }
+
+        // Check diagonals.
+        if (TryGetWinner(grid[0, 0], grid[1, 1], grid[2, 2], out var diag1))
+        {
+            SetWinner(world, ref state, diag1);
+            return;
+        }
+        if (TryGetWinner(grid[0, 2], grid[1, 1], grid[2, 0], out var diag2))
+        {
+            SetWinner(world, ref state, diag2);
+            return;
+        }
+
+        // Check draw.
+        if (state.MoveCount >= 9)
+        {
+            state.Status = GameStatus.Draw;
+            world.MarkModified<GameState>(World.SingletonEntity);
+        }
+    }
+
+    private static bool TryGetWinner(Player a, Player b, Player c, out Player winner)
+    {
+        if (a != Player.None && a == b && b == c)
+        {
+            winner = a;
+            return true;
+        }
+        winner = Player.None;
+        return false;
+    }
+
+    private static void SetWinner(World world, ref GameState state, Player winner)
+    {
+        state.Status = winner == Player.X ? GameStatus.XWon : GameStatus.OWon;
+        world.MarkModified<GameState>(World.SingletonEntity);
+    }
+}