site: replace fumadocs with powderworks-docs - #32

The site was a Next.js application to render twenty-odd pages of
markdown. It is now markdown again, plus three hundred lines of
generator: every directory becomes a section index listing its
children, breadcrumbs come from the path, and nothing at all runs in
the browser.

- content/ holds the same twenty-two pages as plain .md with title,
  description, and order frontmatter; Cards became lists and Callouts
  became blockquotes in the conversion.
- The build calls powderworks-docs (github:PowderworksCode/docs) and
  emits HTML, per-page markdown twins, sitemap.xml, llms.txt, 404.html,
  and theme.css wherever a page needs it.
- The worker still negotiates: Accept: text/markdown on /docs paths
  now serves the twin sitting beside the page.
- Tests keep their teeth: the rules-manifest suite checks the new
  content paths against the scanner, and meta.json validation became
  frontmatter validation.

Search, OG images, and the language-tab component are gone with the
framework; the first can return as a static index when it earns its
complexity back.

powderworks-docs now renders the whole content tree as a sticky
sidebar on non-landing pages, current page marked in the accent
color; the landing stays a single centered column.

The shared generator now renders the index list on every page
including the landing, in the body serif with black text: bold group
headings, current page underlined, no separate nav palette. The docs'
About section is renamed Project so the tree no longer shows two
Abouts. Pages carry previous and next links that walk the same order
the sidebar shows.

The landing carries the Wellcome Collection engraving again, centered
above the prose; strait-face.png sits beside the site name at the top
of the index list and serves as the favicon. The generator now emits
the page title as a single h1 on every page, so leaf pages have a
visible heading for the first time and the landing does not double
its own.

The Introduction wrapper section is gone: guides, reference, and
getting-started are top-level, so the sidebar no longer buries them.
Project keeps philosophy, contributing, and license, and gains
Contact; Privacy is removed outright. The landing stops emitting the
auto-generated listing of every top-level page, which was the block
of four links at its foot.

Internal links move with the pages; the worker negotiates markdown
twins on any path now that twins sit beside everything.