feat: add deferred structural mutations and entity iterators

Implement deferred structural mutations (AddComponent, RemoveComponent,
DestroyEntity) to allow safe modification of the world during
iteration.

Additionally, introduce a new `EntityIterator` API providing zero-
allocation `ref struct` iterators (`Select<T1>`, `Select<T1, T2>`, etc.)
to support manual iteration with early-exit capabilities.

As part of these changes, component access via `GetComponent<T>` during
iteration now automatically marks components as modified.

examples/TicTacToe/Commands/PlaceMarkCommand.cs

@@ -20,20 +20,20 @@ public struct PlaceMarkCommand : ICommand
         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.
+        // Use the iterator API with early-exit via break.
         var query = world.Query().With<Cell>().Without<Mark>().Build();
         Entity? target = null;
 
-        world.ForEach(query, (Entity entity, ref Cell cell) =>
+        using var iter = world.Select<Cell>(query);
+        while (iter.MoveNext())
         {
-            if (cell.Row == row && cell.Col == col)
-                target = entity;
-        });
+            if (iter.Current1.Row == Row && iter.Current1.Col == Col)
+            {
+                target = iter.CurrentEntity;
+                break;
+            }
+        }
 
         if (target == null)
             return; // Cell already occupied or invalid position.

examples/TicTacToe/Systems/RenderSystem.cs

@@ -24,12 +24,14 @@ public class RenderSystem : ISystem
             for (int c = 0; c < 3; c++)
                 grid[r, c] = '.';
 
-        // Fill in placed marks.
+        // Fill in placed marks using the iterator API.
         var markQuery = world.Query().With<Cell>().With<Mark>().Build();
-        world.ForEach(markQuery, (Entity entity, ref Cell cell, ref Mark mark) =>
+        using var markIter = world.Select<Cell, Mark>(markQuery);
+        while (markIter.MoveNext())
         {
-            grid[cell.Row, cell.Col] = mark.Player == Player.X ? 'X' : 'O';
-        });
+            grid[markIter.Current1.Row, markIter.Current1.Col] =
+                markIter.Current2.Player == Player.X ? 'X' : 'O';
+        }
 
         Console.WriteLine();
         Console.WriteLine("  Tic-Tac-Toe");

examples/TicTacToe/Systems/WinCheckSystem.cs

@@ -22,13 +22,14 @@ public class WinCheckSystem : ISystem
         if (state.Status != GameStatus.Playing)
             return;
 
-        // Build a 3×3 grid of marks.
+        // Build a 3×3 grid of marks using the iterator API.
         var grid = new Player[3, 3];
 
-        world.ForEach(Query, (Entity entity, ref Cell cell, ref Mark mark) =>
+        using var iter = world.Select<Cell, Mark>(Query);
+        while (iter.MoveNext())
         {
-            grid[cell.Row, cell.Col] = mark.Player;
-        });
+            grid[iter.Current1.Row, iter.Current1.Col] = iter.Current2.Player;
+        }
 
         // Check rows.
         for (int r = 0; r < 3; r++)