Favicon, notebook test post, expanded About page

[Loyale]

Summary

Three loose ends from the punch list.

• Favicon. Adds files/favicon.svg — a dark abyss-tinted rounded square with two faint light shafts and a bioluminescent "F" monogram (Fraunces-ish serif, #5eead4). Wired into EXTRA_HEAD_DATA via SITE_URL so the absolute href is correct under the /fons_voyage/ subpath. Also exposed as a mask-icon for Safari pinned tabs.
• Notebook test post. posts/testing-the-deep-with-a-notebook.ipynb is a small hand-authored notebook that exercises every style override we shipped: a markdown intro, a pandas DataFrame (rendered HTML + text/plain fallback), a print() stream output, a matplotlib-style figure provided as inline SVG (no real kernel run, hand-built so figure rendering can be verified without a build dependency), and a closing markdown cell pointing back to where the rules live.
• About page. pages/about.md expanded with: cadence and what to expect, follow-along links (RSS, archive, categories), more on the lab and the IGM, an authorship note, ways to contribute corrections, and a brief colophon.

Test plan

• After deploy, confirm the favicon shows in the browser tab and (on Safari) as a pinned-tab mask
• Open /posts/testing-the-deep-with-a-notebook/ and verify:
  • Input cells render with monokai syntax highlighting on the dark code well
  • The DataFrame has a translucent teal header and zebra rows
  • The print() block sits in its own muted box
  • The SVG figure has a light backing so the bars are legible
• Open /pages/about/ and confirm the new sections render with the right typography (Fraunces headings, Inter body, coral links, teal tag chips where applicable)
• Verify RSS/archive/categories links in the About page resolve under the deployed /fons_voyage/ base path

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