Enables goldmark's footnote extension next to GFM (tables, strikethrough and autolinks already work via GFM — only footnotes were missing from the common set).
- a [^name] reference renders as a sup link; [^name]: definitions are appended as a .footnotes block at the end of the post, with backref arrows linking back — both directions plain anchors, no JavaScript
- note bodies are ordinary markdown (links, emphasis, code); external links inside notes get the usual target=_blank rel=noopener
- undefined references stay literal text, so the author notices
- .footnotes CSS: small muted notes block under the existing hr style, backref arrows unaccented; the sup refs use the global link style
- tests pin the rendered shape (refs, notes block, backrefs, markdown inside notes, end-of-document placement, undefined-ref degradation)
- README Components, AGENTS.md, Footnotes section in the markdown-basics showcase post; site rebuilt