refactor: implement auto-tracking for component modifications

Introduce nested batching scopes to automate component dirty-marking
and defer structural mutations.

- Replace manual `MarkModified` with auto-tracking via `RefN` properties
  in `Select` iterators and `GetComponent`/`GetSingleton` calls.
- Implement nested batching scopes: `SystemGroup` (tick level),
  `ISystem.Run`
  (system level), `CommandQueue.ExecuteAll` (command level), and
  `Select`
  (iteration level).
- Update `ISystem` and `ITickedSystem` to use `RunImpl` to separate
  implementation from the batching-aware `Run` extension method.
- Add `ValN` properties to iterators for read-only, non-tracked access.
- Update documentation to reflect the new iteration and batching model.

OECS/WorldQueryExtensions.cs

@@ -46,7 +46,7 @@ public static class WorldQueryExtensions
         return new Select6<T1, T2, T3, T4, T5, T6>(world, query);
     }
 
-    // ── FindEntity / FindEntities ────────────────────────────────────
+    // ── FindEntity ──────────────────────────────────────────────────
 
     public static Entity FindEntity<T1>(this World world, Query<T1> query = default)
         where T1 : struct
@@ -68,33 +68,6 @@ public static class WorldQueryExtensions
         using var iter = new Select3<T1, T2, T3>(world, query);
         return iter.MoveNext() ? iter.Entity : Entity.Null;
     }
-
-    public static List<Entity> FindEntities<T1>(this World world, Query<T1> query = default)
-        where T1 : struct
-    {
-        var list = new List<Entity>();
-        using var iter = new Select1<T1>(world, query);
-        while (iter.MoveNext()) list.Add(iter.Entity);
-        return list;
-    }
-
-    public static List<Entity> FindEntities<T1, T2>(this World world, Query<T1, T2> query = default)
-        where T1 : struct where T2 : struct
-    {
-        var list = new List<Entity>();
-        using var iter = new Select2<T1, T2>(world, query);
-        while (iter.MoveNext()) list.Add(iter.Entity);
-        return list;
-    }
-
-    public static List<Entity> FindEntities<T1, T2, T3>(this World world, Query<T1, T2, T3> query = default)
-        where T1 : struct where T2 : struct where T3 : struct
-    {
-        var list = new List<Entity>();
-        using var iter = new Select3<T1, T2, T3>(world, query);
-        while (iter.MoveNext()) list.Add(iter.Entity);
-        return list;
-    }
 }
 
 // ── Ref struct enumerators ───────────────────────────────────────────