ext/dom: DOMElement::tagName slot init for JIT/AOT (#3326)

[PurHur]

Summary

• Register tagName on DOMElement class metadata and initialize the slot in createElement() / createElementNS() alongside nodeName.
• Fixes JIT/AOT property-slot reads for $el->tagName after createElement() (previously unset while VM used DomNodePropertySupport bridge).

php-src reference

• ext/dom/node.c — dom_node_tag_name_read / element tagName property

PHP implementation

• ext/dom/VmDom.php — class registration + createElement slot init

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/maintainer_gap_dom_element_tag_name_jit.php
# p:p
php bin/jit.php test/repro/maintainer_gap_dom_element_tag_name_jit.php
# p:p
php bin/compile.php -o /tmp/tag test/repro/maintainer_gap_dom_element_tag_name_jit.php && /tmp/tag
# p:p exit=0
php bin/compile.php -o /tmp/dom test/repro/maintainer_gap_dom_create_element_aot.php && /tmp/dom
# p exit=0
php vendor/bin/phpunit test/unit/DomModuleTest.php --filter test_dom_element_set_attribute_on_create_element --no-configuration
# OK'

Closes #3326 (partial — tagName slot parity; load/loadHTML AOT bridge still tracked on issue).

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