perf: cache hash code in QueryDescriptor

fix: notify change buffer when removing components in World

docs: add caution about structural changes during ForEach

fix: prevent multiple disposals in World

src/OECS/QueryDescriptor.cs

@@ -17,10 +17,13 @@ public class QueryDescriptor
     /// </summary>
     public IReadOnlySet<Type> Without { get; }
 
+    private readonly int _hashCode;
+
     internal QueryDescriptor(HashSet<Type> with, HashSet<Type> without)
     {
         With = with;
         Without = without;
+        _hashCode = ComputeHashCode();
     }
 
     /// <summary>
@@ -36,7 +39,9 @@ public class QueryDescriptor
         return With.SetEquals(other.With) && Without.SetEquals(other.Without);
     }
 
-    public override int GetHashCode()
+    public override int GetHashCode() => _hashCode;
+
+    private int ComputeHashCode()
     {
         var hash = new HashCode();
         foreach (var t in With.OrderBy(t => t.GUID.ToString()))

src/OECS/World.cs

@@ -22,6 +22,7 @@ public class World : IDisposable
     private readonly RelationshipIndex _relationships;
     private readonly ChangeBuffer _changes;
     private bool _singletonCreated;
+    private bool _disposed;
 
 #if DEBUG
     // Tracks entities whose components were accessed via GetComponent<T>
@@ -82,6 +83,7 @@ public class World : IDisposable
             {
                 _relationships.OnRemoved(relType, source, entity);
                 _components.Remove(source, relType);
+                _changes.Pending.MarkComponentRemoved(source, relType);
             }
         }
 
@@ -358,6 +360,13 @@ public class World : IDisposable
     /// <summary>
     /// Iterates all entities matching the query, providing ref access to
     /// one component type.
+    ///
+    /// <b>Caution:</b> Adding or removing components (including destroying
+    /// entities) during iteration may cause entities to be skipped or
+    /// processed twice. This is a consequence of the sparse set's
+    /// swap-remove strategy. If you need to make structural changes,
+    /// collect the affected entities first, then apply changes after
+    /// the iteration.
     /// </summary>
     public void ForEach<T1>(
         QueryDescriptor query,
@@ -486,6 +495,9 @@ public class World : IDisposable
 
     public void Dispose()
     {
+        if (_disposed)
+            return;
+        _disposed = true;
         _changes.Dispose();
     }