Stdlib: DOMXPath query/evaluate VM surface (#6066)

[PurHur]

Summary

• Register DOMXPath class with __construct, query(), evaluate(), and registerNamespace() in ext/dom/ (php-src ext/dom/xpath.c)
• PHP-in-PHP XPath evaluator in VmDomXPath.php — descendant //tag, attribute predicates [@id="2"], absolute /path, child axis, count()/string()/boolean() helpers
• No new C runtime logic

php-src reference

• ext/dom/xpath.c — dom_xpath_query, dom_xpath_evaluate, dom_xpath_register_ns

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro-maintainer/parity_domxpath.php'
# ok

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter dom_xpath_query'
# OK (3 tests)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter DomModuleTest::test_dom_xpath_query_attribute_predicate'
# OK (1 test)

Closes #6066

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