Render Markdown to standalone

Key features

• md_html_render — a tool the model can call.
• The client half loads as a plain script with no bundler, so marked and markdown-it are not available to it.
• tests/render.test.cjs extracts the client renderer from the browser bundle and runs it.
• The / in the path bar.
• Give it Markdown, get back a complete, self-contained HTML document, optionally written to disk.