Fix AOT SimpleXMLElement::registerXPathNamespace for xpath() (#27534)

[PurHur]

Summary

• Fold SimpleXMLElement::registerXPathNamespace() on the host php-src tree during user-script AOT so a subsequent compile-time xpath() sees registered prefixes (matches VM/JIT/Zend).
• Wire JIT proxy + spine coverage for new lowering helpers.
• Refresh honest gen-0 argv driver after lowering-source drift from this change.

php-src reference

• ext/simplexml/simplexml.c — registerXPathNamespace / xpath
• ext/simplexml/sxe.c — namespace registration on SXE handle

PHP implementation

• ext/simplexml/JitSimpleXmlUserScript.php — tryRegisterXPathNamespace()
• ext/simplexml/JitSimpleXmlRegisterXPathNamespace.php + lib/JIT/Call/SimpleXMLElementRegisterXPathNamespace.php

Verification

export PHP_COMPILER_LLVM_PATH="$(pwd)/.llvm"

# Issue repro (VM/JIT/AOT) — 3× AOT runs
php bin/vm.php /tmp/issue_sxe_register_xpath_ns.php   # count=1 val=1
php bin/jit.php /tmp/issue_sxe_register_xpath_ns.php  # count=1 val=1
for i in 1 2 3; do php bin/compile.php -o /tmp/sxe_$i /tmp/issue_sxe_register_xpath_ns.php && /tmp/sxe_$i; done
# all: count=1 val=1

make north-star5-verify-fast   # OK (fast)
php script/check-selfhost-spine-coverage-sync.php  # OK 7008/7008
php script/check-bootstrap-gen0-manifest-sync.php  # OK

Not covered: full north-star5-verify --strict, compliance suite shards, differential sweep (compile-time fold only; VM/JIT already matched).

Closes #27534

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