Return empty document fragment from appendChild

[oiahoon]

Summary

• return an empty DocumentFragment from appendChild() instead of propagating the native false result
• add a regression test that verifies object identity and confirms the parent remains unchanged

Why

PHP's native DOMNode::appendChild() returns false for an empty document fragment. The wrapper's strict return type turns that value into a TypeError, while the DOM API is expected to return the appended fragment.

Testing

• composer test (960 tests, 3829 assertions; PHPStan, PHPCS, and PHPMD passed)
• composer validate --strict --no-check-publish
• focused NodeTest and DocumentFragmentTest suite (65 tests, 95 assertions)
• manual empty and non-empty fragment checks

Closes #458

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 0 duplication

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[g105b]

Looks perfect.

[g105b]

Thank you @oiahoon !