Stdlib: DOMText::splitText() text node split (#17513)

[PurHur]

Summary

• Implement DOMText::splitText(int $offset) in ext/dom/TextSplitText.php + VmDom::textSplitText() (php-src ext/dom/text.c)
• Fix syncNodeLinks() to refresh previousSibling/nextSibling when a parent already has a cached childNodes list (required for split + sibling mutation paths)

php-src reference

• ext/dom/text.c — dom_text_split_text()
• ext/dom/characterdata.c — shared CharacterData mutation

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_dom_split_text.php'
# ok

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter DomModuleTest::test_dom_text_split_text'
# OK (1 test)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter dom_text_split_text'
# OK (3 tests — VM + JIT compliance)

Closes #17513

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