Node::compact_elements_to we rely on no-op for one case, but comment suggests that is an error case

`Node::compact_elements_to` --- the if statement has a comment that when true, we should have an error (compacting something that can't be compacted)

if we change this to an error, tests fail.

It might be that this is just a pass through and should be a no-op, but we should understand deeper.

Start at the Node::compact_elements_to entry point and trace the condition whose comment describes an error case. Check the existing tests that fail when this behavior changes, then determine whether the condition should remain a no-op or report an error. Done means the behavior and comment agree and the affected tests pass.

Written by the indexing model from the issue text.