CVE-2026-83609: xmldom: Creation-time XML Name/QName validation is bypassable via an embedded line terminator, allowing injection on the default serialization path

An embedded line terminator bypasses xmldom’s always-on, WHATWG-mandated creation-time name validation. createElementNS, createAttributeNS, createDocumentType, and createAttribute should reject a malformed qualified name with InvalidCharacterError, but a name whose first line is well-formed slips through and enters the DOM. On serialization it is emitted verbatim, so the characters after the line terminator inject markup into the output. The injection reaches the default serialization path, and enabling requireWellFormed does not prevent it.