Fix AOT DOM id-map rebind, isId(), getElementById disconnected guard

[PurHur]

Summary

• AOT: loadXML DTD ATTLIST ID / xml:id leaves Attr::isId() false — Zend true (re-#34696/#29884, ext/dom/attr.c) #34821 — DOMAttr::isId() consults compile-time id-bearing flags before the user-function global path.
• Regression: AOT DOMDocument::getElementById null after replaceChild same id + setIdAttribute — Zend/VM null, AOT finds node (re-#25274, ext/dom/node.c) #29694 — replaceChild clears detached link slots; id map uses receiver attrs (not global Attr cache); map fallback returns null for disconnected nodes; setIdAttribute skips cache when id is already registered.
• Regression: DOMDocument::getElementById() stale after setAttribute/removeAttribute id (ext/dom/document.c) #19870 — loadHTML setAttribute('id', …) rebind uses getElementById pairing + parsed id; single-slot cache stays authoritative (no isConnected filter on cache hits — compile-time temps lack parent slots).

php-src: ext/dom/attr.c (dom_attr_is_id), ext/dom/document.c (dom_get_element_by_id), ext/dom/node.c (detach clears links).

Test plan

./script/aot-smoke.sh
# aot-smoke: 8 passed, 0 failed

./script/phpunit.sh --filter 'Issue34821DomDtdIsIdAotTest|Issue29694DomGetElementByIdReplaceChildAotTest|DomGetElementByIdSetAttributeIdAotTest|DomReplaceChildSetIdAttributeIdMapAotTest|DomSetAttributeIdRebind35321AotTest'
# OK (6 tests, 25 assertions)

./script/docker-exec.sh -- bash -lc './test/repro/maintainer_gap_dom_getelementbyid_setattribute_id && ./test/repro/maintainer_gap_dom_getelementbyid_replacechild_same_id'
# before=x / after_b=1 / after_a=0 / … ; null / c

Closes #34821
Closes #29694
Closes #19870

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