Stdlib: DOMElement::setIdAttribute() missing — getElementById without DTD returns null (ext/dom/node.c) #14493

Problem

DOMElement::setIdAttribute() is undefined on this compiler. Zend registers the method so callers can mark an attribute as an XML ID and use DOMDocument::getElementById() without a DTD ID declaration.

Today:

fail: DOMElement::setIdAttribute() undefined

After setIdAttribute('id', true), Zend resolves getElementById('target'); VM always returns null for undeclared-ID documents.

php-src reference

• ext/dom/node.c — dom_element_set_id_attribute(), ID table registration
• ext/dom/php_dom.c — getElementById lookup

Implement in ext/dom/ (PHP-in-PHP); no new C runtime logic.

Repro

./script/bootstrap-selfhost-gate.sh link  # host gate
php test/repro/maintainer_gap_dom_set_id_attribute.php          # zend=0
php bin/vm.php test/repro/maintainer_gap_dom_set_id_attribute.php  # vm=1

Done when (php-src-strict)

• method_exists($el, 'setIdAttribute') true on DOMElement
• setIdAttribute('id', true) + getElementById('target') returns the element (no DTD)
• Targeted compliance guard under test/compliance/ or test/repro/

Refs #6189, #14455, #1492