AOT: Document childNodes + before/after comment match Zend (#34160)

[PurHur]

Summary

• Fix thin-AOT DOMDocument::$childNodes to read/write DOMDocument layout (was always Element), so LiveSlots after ChildNode::before/after / insertBefore are visible.
• Seed loadXML Document firstChild/lastChild/childNodes with the root; walk Document children in document-wide saveXML (php-src xmlDocDumpMemory).
• Owner-aware DOMNodeList::item() uses Document-aware firstChild; skip UserScript item(0) pinned-root shortcut after tree mutations.

Closes #34160

php-src

• ext/dom/php_dom.c — dom_child_node_before / dom_add_prev_sibling
• ext/dom/document.c — saveXML → xmlDocDumpMemory

Test plan

• ./script/aot-smoke.sh → 8 passed, 0 failed
• ./script/phpunit.sh --filter Issue34160DomDocumentChildNodeBeforeCommentAotTest → OK (2 tests, 6 assertions)
• AOT repro matches Zend for before/after/insertBefore comment on Document
• Regression: AOT: ChildNode::after() append-tail drops sibling from saveXML — LiveSlots see it (ext/dom/php_dom.c) #34136 after append-tail still <r><b/><c/></r>

Verify transcript

aot-smoke: 8 passed, 0 failed

PHPUnit … OK (2 tests, 6 assertions)

AOT issue_34160:
<?xml version="1.0"?>
<!--x-->
<r/>
len=2
item0=#comment
item1=r
…
len3=2

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