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.