Stdlib: DOMNode replaceChild/insertBefore/removeChild missing — tree mutation fatal (ext/dom/node.c) #14394

Summary

Classic DOM Level 2 tree mutation on DOMNode — replaceChild(), insertBefore(), and removeChild() — fatals on the VM with undefined method on DOMElement. Zend mutates child order and returns removed/replaced nodes.

Distinct from #14380 append()/prepend() (PHP 8 living-standard helpers); these legacy methods are still required for php-src parity and existing XML tooling.

php-src reference

• ext/dom/node.c — dom_node_replace_child, dom_node_insert_before, dom_node_remove_child
• ext/dom/node.stub.php

PHP implementation target

• ext/dom/VmDom.php + node wrappers (PHP-in-PHP; thin libxml ABI only)

Repro (php-src-strict)

./script/docker-exec.sh -- php test/repro/maintainer_gap_dom_node_tree_mutation.php          # Zend: ok
./script/docker-exec.sh -- php bin/vm.php test/repro/maintainer_gap_dom_node_tree_mutation.php  # VM: fatal replaceChild

Done when

• replaceChild / insertBefore / removeChild match Zend child order and return values (php-src-strict)
• Wrong-document and not-a-child errors match php-src
• test/repro/maintainer_gap_dom_node_tree_mutation.php green on VM
• PHPUnit guard; VM/JIT/AOT agree when both in scope