Implement DOMDocumentFragment createDocumentFragment/appendChild (#6317)

[PurHur]

Summary

• Register DOMDocumentFragment and DOMDocument::createDocumentFragment() in PHP-in-PHP ext/dom/.
• Implement fragment appendChild() and merge fragment children into parent nodes without a wrapper element (php-src ext/dom/php_dom.c).
• Fix DOMDocument::appendChild() when a document element already exists: append siblings to the document instead of redirecting into the root element.

Verification

./script/docker-exec.sh -- bash -lc 'vendor/bin/phpunit --filter dom_document_fragment'
# OK (2 tests, 2 assertions) — VM + JIT

./script/docker-exec.sh -- bash -lc 'php bin/vm.php -r "var_export(class_exists(\"DOMDocumentFragment\")); echo PHP_EOL;"'
# bool(true)

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/dom_document_fragment.php'
# class ok / instance ok / fragment empty after merge / saveXML shows <root><item/></root>

Closes #6317

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