php-in-php: ext/libxml error buffer — LibXMLError without runtime/*.c (#6058)

[PurHur]

Summary

• Add ext/libxml/ module with libxml_use_internal_errors(), libxml_get_errors(), libxml_clear_errors(), and LibXMLError class — pure PHP ring buffer per php-src ext/libxml/libxml.c.
• Wire xml_parse() malformed-input probe (ext/xml/VmXml.php) to record errors via VmLibxml::handleError() for DOM/SimpleXML loader integration later.
• C shrink: 0 lines added to runtime/ or lib/AOT/runtime/; deleted unused ext/xml/XmlFunction.php stub (29 lines).

php-src reference

• ext/libxml/libxml.c — internal error toggle + buffer
• ext/libxml/libxml_error.c — LibXMLError object fields

PHP implementation

Layer	Path
VM buffer	ext/libxml/VmLibxml.php
Builtins	ext/libxml/libxml_*.php
XML probe	ext/xml/VmXml.php

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php -r "
echo function_exists(\"libxml_use_internal_errors\") ? \"yes\" : \"no\", PHP_EOL;
echo class_exists(\"LibXMLError\") ? \"yes\" : \"no\", PHP_EOL;
"'
# yes / yes

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit test/unit/LibxmlInternalErrorsTest.php'
# OK (4 tests, 15 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter libxml_internal_errors'
# OK (2 tests) — VM + JIT compliance

Closes #6058

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