refactor: behaviour collector Count and indexer accessors added

Engine.Core/BehaviourCollector.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Collections;
 using System.Collections.Generic;
 
@@ -14,7 +13,6 @@ public class BehaviourCollector<T> : IBehaviourCollector<T> where T : class
 
     protected readonly List<T> behaviours = new(32);
 
-    public IReadOnlyList<T> Behaviours => behaviours;
     public IUniverse Universe { get; private set; } = null!;
 
     public BehaviourCollector() { }
@@ -96,4 +94,7 @@ public class BehaviourCollector<T> : IBehaviourCollector<T> where T : class
         OnUnassigned?.Invoke(this);
         return true;
     }
+
+    public int Count => behaviours.Count;
+    public T this[System.Index index] => behaviours[index];
 }