php-in-php: ext/dom DOMImplementation factory without host DOM (#6140)

[PurHur]

Summary

• Add ext/dom/ module with PHP-owned DOM tree state (DomRegistry, VmDom) — no host \DOM* delegation and no new runtime/*.c symbols
• Implement v1 factory surface: DOMImplementation::createDocumentType, createDocument, hasFeature, and DOMDocument::saveXML + formatOutput property
• Wire module in lib/Runtime.php; compliance PHPT + unit tests assert Zend parity on the issue repro

php-src reference

• ext/dom/php_dom.c — dom_implementation_create_document, dom_implementation_create_document_type
• PHP implementation: ext/dom/VmDom.php, ext/dom/Implementation*.php

C runtime shrink

No C added. lib/AOT/Linker.php unchanged (still only phpc_progress.c thin ABI).

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/unit/DomModuleTest.php test/compliance/DOMImplementationVMTest.php
php bin/vm.php test/repro-maintainer/dom_implementation_6140.php
'

Output (green):

OK (2 tests, 7 assertions)
OK (1 test, 1 assertion)
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html/>
hasFeature XML 2.0: 1
hasFeature HTML 2.0: 0
hasFeature XBL 2.0: 0

Closes #6140

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