Stdlib: DOMNode::replaceChild accept Text/Comment children (#21976)

[PurHur]

Summary

• Fix DOMNode::replaceChild() rejecting non-Element children (createTextNode → Hierarchy request error) while removeChild+appendChild worked.
• Align with php-src ext/dom/node.c dom_node_replace_child: allow tree-mutation children (Text/CDATA/Comment/PI/EntityRef/Element); expand DocumentFragment in place; update documentElement only when replacing with/away from an Element.
• Compliance + repro under test/compliance/cases/dom/ and test/repro/.

Closes #21976

php-src reference

• ext/dom/node.c — dom_node_replace_child / hierarchy checks

PHP implementation

• ext/dom/VmDom.php — replaceChild()

Test plan

• VM repro green
• JIT repro green
• Compliance dom_node_replacechild_text.phpt (VM+JIT)
• Related replaceChild regressions (Regression: DOMNode::replaceChild(createElement(...), childNodes->item(...)) — temp $child null / live list (ext/dom/node.c, re-#21171) #21182, setIdAttribute idmap, ancestor hierarchy) still green on VM
• AOT: pre-existing helper gaps (createTextNode / toObject on DOM JIT helpers) — out of issue done-when (VM+JIT)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/issue_21976_dom_replacechild_text.php'
# before=abcdef / after=ZZ / workaround=YY / attr DOMException

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/jit.php test/repro/issue_21976_dom_replacechild_text.php'
# same

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter dom_node_replacechild_text'
# VM+JIT compliance pass

Pre-claim gates (master): inventory OK 6335/6335; north-star5-fast FAIL gen-0 mismatch #8713 (pre-existing).

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