Respect SameValue on Element.classList

`Element.classList` has `sameObject` specified, but the implementation returns a new object each time.

Start at the Element.classList implementation and inspect how repeated property access creates or returns the object. Verify the behavior against the specified SameValue requirement, then add or update coverage for repeated Element.classList reads and run the relevant Go tests. Done means repeated accesses return the same object.

Written by the indexing model from the issue text.