GHSA-r758-8hxw-4845: justhtml: Mutation XSS with custom foreign-namespace sanitization policies

A parser-differential / mutation XSS issue was found in justhtml when using a custom sanitization policy that preserves foreign namespaces such as SVG or MathML.

Under these custom settings, specially crafted input could sanitize into HTML that looked safe at first, but became unsafe when parsed again by a browser or another HTML parser.