Adopting a node into another document preserves the node's dirty style
flags, but the destination ancestor chain never sees them propagate. If
a style update is already pending in the new document, it can skip the
adopted subtree entirely.
Snapshot the subtree and child dirty bits before set_document() updates
m_document, then walk the new ancestor chain and re-mark
child_needs_style_update so the pending restyle still descends into the
adopted subtree.
