Stdlib: xml_parse_into_struct() SAX struct builder (#3494)

[PurHur]

Summary

• Implement xml_parse_into_struct() in ext/xml/ with PHP-in-PHP SAX-shaped struct walking (VmXmlStructBuilder).
• Register by-ref parameter indices [2, 3] in BuiltinByRefParams so &$values / &$index writeback matches Zend.
• Add repro, compliance .phpt, and unit tests.

php-src reference

• ext/xml/xml.c — xml_parse_into_struct, xml_startElementHandler, xml_endElementHandler, xml_characterDataHandler

PHP implementation

• ext/xml/xml_parse_into_struct.php, ext/xml/VmXmlStructBuilder.php, ext/xml/VmXml.php::parseIntoStruct()
• lib/BuiltinByRefParams.php — xml_parse_into_struct → [2, 3]

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/unit/XmlParseIntoStructTest.php --no-configuration
php bin/vm.php test/repro/maintainer_gap_xml_parse_into_struct.php
php bin/vm.php -r "
\$vals = []; \$idx = [];
xml_parse_into_struct(xml_parser_create(), \"<root attr=\\\"v\\\">text<child/></root>\", \$vals, \$idx);
var_export(\$vals);
"'

Output (matches Zend case-folded tags/attributes):

PHPUnit ... OK (4 tests, 11 assertions)
ok
array (
  0 => array ('tag' => 'ROOT', 'type' => 'open', 'level' => 1, 'attributes' => array ('ATTR' => 'v'), 'value' => 'text'),
  1 => array ('tag' => 'CHILD', 'type' => 'complete', 'level' => 2),
  2 => array ('tag' => 'ROOT', 'type' => 'close', 'level' => 1),
)

Closes #3494

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