DOM: ID map on detach + setIdAttribute first-wins (#25274, #25275)

[PurHur]

Summary

• Match libxml/php-src 8.2: removeChild/replaceChild no longer clear document elementIds — IDs stay bound to the detached node; getElementById already returns null for disconnected entries.
• registerElementId / setIdAttribute use xmlAddID first-wins semantics (no overwrite; failed add leaves isId unset).
• Closes Regression: DOMDocument::getElementById() null after replaceChild same id + setIdAttribute — Zend null, VM finds node (re-#23999, ext/dom/node.c) #25274 and Regression: DOMDocument::getElementById() with duplicate setIdAttribute ids — Zend first wins, VM last (ext/dom/document.c) #25275 (cohesive ID-map subtree).

php-src reference: ext/dom/document.c getElementById + ext/dom/element.c php_set_attribute_id → xmlAddID/xmlRemoveID.

Test plan

• Zend vs VM repros:

  ./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
  php test/repro/maintainer_gap_dom_getelementbyid_replacechild_same_id.php
  php bin/vm.php test/repro/maintainer_gap_dom_getelementbyid_replacechild_same_id.php
  php test/repro/maintainer_gap_dom_getelementbyid_duplicate_setidattribute.php
  php bin/vm.php test/repro/maintainer_gap_dom_getelementbyid_duplicate_setidattribute.php'

  Output (both): null / c and a
• JIT: same repros via php bin/jit.php — match
• vendor/bin/phpunit --filter 'dom_getelementbyid|dom_setidattribute|…' — 26 OK
• Related Regression: DOMDocument::getElementById() after setIdAttribute on detached node + replaceChild — Zend null, VM finds node (ext/dom/node.c) #23999 detached replaceChild still green
• AOT user-script setIdAttribute still unbound (Call to undefined method object::setidattribute()) — pre-existing; not in this change’s scope

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