feat: add simplified Select extension methods to EntityIterator

Add overloads for Select that allow iterating over components without
manually building a QueryDescriptor, simplifying usage in systems.

examples/TicTacToe/Systems/RenderSystem.cs

@@ -18,8 +18,7 @@ public class RenderSystem : ISystem
                 grid[r, c] = '.';
 
         // Fill in placed marks using the iterator API.
-        var markQuery = world.Query().With<Cell>().With<Mark>().Build();
+        using var markIter = world.Select<Cell, Mark>();
-        using var markIter = world.Select<Cell, Mark>(markQuery);
         while (markIter.MoveNext())
         {
             grid[markIter.Current1.Row, markIter.Current1.Col] =

examples/TicTacToe/Systems/WinCheckSystem.cs

@@ -18,8 +18,7 @@ public class WinCheckSystem : ISystem
         // Build a 3×3 grid of marks using the iterator API.
         var grid = new Player[3, 3];
 
-        var query = world.Query().With<Cell>().With<Mark>().Build();
+        using var iter = world.Select<Cell, Mark>();
-        using var iter = world.Select<Cell, Mark>(query);
         while (iter.MoveNext())
         {
             grid[iter.Current1.Row, iter.Current1.Col] = iter.Current2.Player;

src/OECS/EntityIterator.cs

@@ -20,6 +20,17 @@ public static class EntityIterator
         return new Select1<T1>(world, query);
     }
 
+    /// <summary>
+    /// Returns an iterator over entities that have a component of type
+    /// <typeparamref name="T1"/>. No <c>Without</c> filter is applied.
+    /// </summary>
+    public static Select1<T1> Select<T1>(
+        this World world)
+        where T1 : struct
+    {
+        return new Select1<T1>(world, new QueryDescriptor(new HashSet<Type>(), new HashSet<Type>()));
+    }
+
     /// <summary>
     /// Returns an iterator over entities matching the query, with ref access
     /// to two component types.
@@ -31,6 +42,18 @@ public static class EntityIterator
         return new Select2<T1, T2>(world, query);
     }
 
+    /// <summary>
+    /// Returns an iterator over entities that have both components of type
+    /// <typeparamref name="T1"/> and <typeparamref name="T2"/>.
+    /// No <c>Without</c> filter is applied.
+    /// </summary>
+    public static Select2<T1, T2> Select<T1, T2>(
+        this World world)
+        where T1 : struct where T2 : struct
+    {
+        return new Select2<T1, T2>(world, new QueryDescriptor(new HashSet<Type>(), new HashSet<Type>()));
+    }
+
     /// <summary>
     /// Returns an iterator over entities matching the query, with ref access
     /// to three component types.
@@ -41,6 +64,18 @@ public static class EntityIterator
     {
         return new Select3<T1, T2, T3>(world, query);
     }
+
+    /// <summary>
+    /// Returns an iterator over entities that have all three components of type
+    /// <typeparamref name="T1"/>, <typeparamref name="T2"/>, and
+    /// <typeparamref name="T3"/>. No <c>Without</c> filter is applied.
+    /// </summary>
+    public static Select3<T1, T2, T3> Select<T1, T2, T3>(
+        this World world)
+        where T1 : struct where T2 : struct where T3 : struct
+    {
+        return new Select3<T1, T2, T3>(world, new QueryDescriptor(new HashSet<Type>(), new HashSet<Type>()));
+    }
 }
 
 /// <summary>