Markdown pipeline: marked + sanitize-html with XSS tests

marked does not sanitise; the sanitiser is an explicit dependency in one shared module.

• renderMarkdown: parse, sanitise (prose tags + pre/code; images from assets.jamesmddoyle.com only), return HTML
• Used by projects and articles only; the guestbook is plain text and never touches this pipeline
• Unit tests: script injection, event handlers, out-of-allowlist image sources