Implied End Tags not Handled Correctly

While working on #109 I noticed that one can create a bit of an odd situation when trying to rewrite tags in the presence of optional end tags.

Take this list:

```html

• a
  
• b
  

When I select `li` and change the tag to `div` I end up with the following output:

```html

a


b



```

This tests shows the issue

```rust

#[test]
fn optional_end_tags() {
let output = rewrite_element(b"

• a
• b

", UTF_8, "li", |el| {
el.set_tag_name("div").unwrap();
});
assert_eq!(output, "

a
b

");
}
```

I believe the system would need to emit "virtual" end tags for when implicitly closed tags are encountered. I believe the system also does not handle this tag stack correctly otherwise which probably should confuse the css selector as well.

Start with the optional_end_tags test and the rewrite_element call shown in the issue, then inspect how the tag stack handles implicitly closed li elements and how CSS selection sees them. Done means rewriting li to div produces correctly balanced output while preserving the surrounding ul structure and the test passes.

Written by the indexing model from the issue text.