xmlns-prefixed attribute on root element is lost

<!-- before -->
<svg xmlns="http://www.w3.org/2000/svg" xmlns:sodipodi="http://inkscape.sourceforge.net/DTD/sodipodi-0.dtd">
    <sodipodi:namedview>
        ...
    </sodipodi:namedview>

    <path d="..." sodipodi:nodetypes="cccc"/>
</svg>

<!-- after: xmlns:sodipodi moved to child -->
<svg xmlns="http://www.w3.org/2000/svg">
    <sodipodi:namedview xmlns:sodipodi="http://inkscape.sourceforge.net/DTD/sodipodi-0.dtd">
        ...
    </sodipodi:namedview>

    <path d="..." sodipodi:nodetypes="cccc"></path>
</svg>

<!-- before -->
<svg xmlns="http://www.w3.org/2000/svg" xmlns:sodipodi="http://inkscape.sourceforge.net/DTD/sodipodi-0.dtd">
    <path d="..." sodipodi:nodetypes="cccc"/>
</svg>

<!-- after: xmlns:sodipodi removed entirely -->
<svg xmlns="http://www.w3.org/2000/svg">
    <path d="..." sodipodi:nodetypes="cccc"></path>
</svg>

Originally posted by @noahbald in https://github.com/servo/html5ever/issues/538#issuecomment-2506784772

Start by reproducing the two XML examples in the issue and tracing how html5ever handles namespace declarations and prefixed attributes during parsing and serialization. Done means the root xmlns:sodipodi declaration is preserved and the resulting prefixed elements and attributes remain correctly associated with that namespace.