XML: create text nodes before mutation

XML: create text nodes before mutation

Previously, text updates encoded content manually and then passed it to
libxml2. This duplicated libxml2 escaping rules and allocated the
encoded buffer before creating the text node.

The fix is to create a libxml2 text node from the unencoded value before
replacing the copied node's children. libxml2 now owns escaping, and a
failure leaves the original XML tree unchanged.