A `<br>` is a void element but `resolveSingleChildTextTarget` treated a lone
`<br>` child as the element's text target. So `getOwnText(<h1>A<br>B</h1>)`
read the `<br>`'s (empty) textContent → `text: null`, which consumers surface
as "not editable", and `setOwnText` wrote into the `<br>`, corrupting it
(serialized as invalid `</br>`).
Exclude void elements from the single-child text target; read `<br>` as "\n";
and rebuild the text/`<br>` run from the newline-separated value on write,
reusing existing `<br>` nodes so their identity (data-hf-id) survives an
in-place edit.
Hblee