DOM: scope getElementById LLVM cache by document (#20830)

[PurHur]

Summary

• AOT getElementById() consulted DomUserScriptElementCacheLlvm by id string only; a prior setIdAttribute() on document A made getElementById('y') on document B return the wrong element (setid_leak) after importNode().
• lookupObject() now requires GLOBAL_DOC to match the requesting document when a receiver is provided.
• Adds DomImportNodeGetElementByIdReindex20830AotTest covering test/repro/maintainer_gap_dom_importnode_getelementbyid_reindex.php.

Root cause

php-src ext/dom/node.c — setIdAttribute does not survive importNode (only copyable libxml ID atypes do). VM path was correct; thin-AOT user-script cache ignored document scope.

Test plan

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

./script/docker-exec.sh -- bash -lc 'f=test/repro/maintainer_gap_dom_importnode_getelementbyid_reindex.php; php bin/vm.php "$f"; tmp=/tmp/dom.bin; php bin/compile.php -o "$tmp" "$f" && "$tmp"'
# ok (VM + AOT)

./script/phpunit.sh --filter DomImportNodeGetElementByIdReindex20830AotTest
# OK (1 test, 5 assertions)

Closes #20830

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