CVE-2026-83616: xmldom: Processing Instruction Target Injection Bypasses requireWellFormed

Document.createProcessingInstruction() in @xmldom/xmldom performs no validation on the target parameter. The requireWellFormed: true serializer option validates only for : in the target and a case-insensitive xml prefix, but does not check for > characters. A > in the target breaks the processing instruction boundary (<?...?>), allowing injection of arbitrary content into the serialized XML output.