Stdlib: SimpleXML methods/import excess argc → ArgumentCountError (#30828)

[PurHur]

Summary

• SimpleXMLElement methods (children, attributes, xpath, registerXPathNamespace, addChild, addAttribute, getName, count, getNamespaces, getDocNamespaces, asXML/saveXML, __construct, __toString, hasChildren, getChildren) now throw Zend-shaped ArgumentCountError on excess user argc instead of silently accepting extra arguments.
• Same for simplexml_load_string/simplexml_load_file (at most 5), simplexml_import_dom (at most 2), and dom_import_simplexml / Dom\import_simplexml (exactly 1).
• JIT/AOT instance-method proxies emit a catchable ArgumentCountError (php-src-strict; php-src ext/simplexml/simplexml.stub.php + ext/dom/php_dom.stub.php).

Closes #30828

Test plan

• Zend vs VM repro identical
• ./script/phpunit.sh --filter SimpleXmlExcessArgc30828 — VM + JIT OK (2 tests)
• php script/bootstrap-inventory.php --check — OK 7430/7430
• php script/check-selfhost-spine-coverage-sync.php — OK
• Existing SimpleXML legal-arity repros (maintainer_gap_simplexml_instance_methods, children_attributes, simplexml_load_string_basic) still exit 0

Verification

php test/repro/maintainer_gap_simplexml_excess_argc_30828.php
php bin/vm.php test/repro/maintainer_gap_simplexml_excess_argc_30828.php
# messages match Zend 8.2.32; legal path prints ok=r,3,r

./script/phpunit.sh --filter SimpleXmlExcessArgc30828
# OK (2 tests, 2 assertions)

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