[PurHur]

Summary

indexElementIdsRecursive() now treats plain id attributes as ID-map entries when the document was loaded via loadHTML(), matching php-src HTML getElementById() behavior.

php-src reference: ext/dom/document.c — HTML document id table

Closes #15251

Test plan

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_dom_loadhtml_get_element_by_id.php'
# ok

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_dom_get_element_by_id.php'
# null / missing_null (XML without DTD id — unchanged)

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_dom_get_element_by_id_dtd.php'
# dtd_ok (DTD ID path unchanged)

./script/docker-exec.sh -- bash -lc 'php vendor/bin/phpunit --filter document_loadhtml_get_element_by_id'
# OK (2 tests)

php bin/compile.php -l test/repro/maintainer_gap_dom_loadhtml_get_element_by_id.php
# AOT lint clean

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