Full-featured, WCAG 2.2 AAA-capable casket-ssg (#34)

## Summary
Turns casket-ssg into a full-featured static-site generator and makes
its default output WCAG 2.2 AAA-capable, plus repo cleanup.

## Engine (src/CasketGnosis.hs)
- Recursive multi-page build; frontmatter + draft-skipping
- Pandoc rich markdown (anchors, pipe tables, footnotes, task lists,
strikethrough, smart punctuation, raw HTML) + syntax highlighting →
`/assets/highlight.css`
- File layouts + partials (`{{> name}}` / `{{include:}}`) +
`{{toc}}`/`{{nav}}`/`{{site.*}}`; `site.conf`
- Collections + tag pages; `sitemap.xml` + Atom `feed.xml`; clean URLs

## Accessibility (WCAG 2.2 AAA-capable, engine-owned surface)
- Contrast-verified ≥7:1 palette (light+dark), skip link, labelled
landmarks, `:focus-visible`, reduced-motion, reflow, ≥44px targets
- Generated tables get `<th scope="col">`
- Verified **vnu-clean** (W3C Nu validator); see
`docs/ACCESSIBILITY-CHECKLIST.adoc`

## Cleanup
- Removed dead top-level `Casket.hs` (cabal builds `CasketGnosis.hs`)
- Removed duplicate, mislabelled `README.md` (AsciiDoc-in-`.md` with a
wrong PMPL-1.0 badge); kept `README.adoc` + added Sponsor badge

All commits signed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath