fix: clean up _transformHashSet in RemoveHandle to allow handle re-creation

RemoveHandle did not remove transforms from _transformHashSet, causing
subsequent CreateHandle calls for the same transform to fail silently
with "already has a handle" warning.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: manaporkun

Packages/com.orkunmanap.runtime-transform-handles/Runtime/Scripts/TransformHandleManager.cs

@@ -280,6 +280,11 @@ public void RemoveHandle(Handle handle)
             _handleGroupMap.Remove(handle);
             handle.Disable();
 
+            foreach (var target in group.Transforms)
+            {
+                _transformHashSet.Remove(target);
+            }
+
             var groupGhost = group.GroupGhost;
             if (groupGhost != null)
             {