Work around C14N duplicate xmlns bug on libxml 2.9.14

[veewee]

Summary

• canonicalize(): round-trip through saveXml() before calling C14N() to normalize namespace declarations that get corrupted in DOM-manipulated documents
• rename_element_namespace(): remove stray Attr::rename() call that ran on a detached attribute node after removeAttributeNode()

Problem

C14N() on DOM-manipulated documents (e.g. after optimize_namespaces()) produces duplicate xmlns declarations when the element has a default xmlns and non-xmlns attributes. On libxml 2.9.14 (Ubuntu 24.04), parsing the invalid C14N output hangs or crashes with double free.

Minimal reproducer:

$doc = Dom\XMLDocument::createFromString('<root xmlns="urn:a" attr="val"/>');
$doc->documentElement->setAttributeNS("http://www.w3.org/2000/xmlns/", "xmlns:ns1", "urn:a");
echo $doc->C14N();
// Output: <root xmlns="urn:a" xmlns:ns1="urn:a" attr="val" xmlns:ns1="urn:a"></root>
//         ^ xmlns:ns1 appears TWICE

This caused comparable() (which calls optimize_namespaces() then canonicalize()) to hang on CI with libxml 2.9.14.

Test plan

• New test CanonicalizeTest::test_it_can_canonicalize_after_dom_manipulation passes
• New ComparableTest data case default-xmlns-with-attributes passes
• Full test suite passes on libxml 2.9.14 (tested via Docker docphpro/php84)
• Full test suite passes on libxml 2.9.13 (tested locally on macOS)