CVE-2026-83606: xmldom PI grammar regex ReDoS: quadratic backtracking on unterminated processing instructions

@xmldom/xmldom’s processing-instruction (PI) grammar regex exhibits quadratic-time backtracking (ReDoS) when parsing an unterminated processing instruction. A single small XML document containing <? + a target + a long run of whitespace and no closing ?> forces the regular expression engine into O(n²) work, stalling the Node.js event loop. The input is parsed with DOMParser.parseFromString under default options, so it is reachable from unauthenticated, network-delivered XML (SOAP/SAML, webhooks, uploads, XML APIs).