docs(site): add CUSTOM_ID anchors, fix links, refresh media

docs(site): add CUSTOM_ID anchors, fix links, refresh media

- Add kebab-case :CUSTOM_ID: slugs to headings across all index.org
  pages so exported HTML gets stable, readable anchor ids
  (index.html#fractals instead of org-generated ids)
- Replace fragile [[id:UUID]] in-page links with slug-based [[#slug]]
  links (LPT driver API table, root index overview)
- Fix "Back to BASIC applications collection" links pointing one
  directory level too shallow in category pages
- Fix Miscellaneous 3D demos link pointing to the Space page, and the
  duplicated :ID: UUID shared by the Games and Math sections
- Replace stale .png/.webm media references with recaptured
  "- Screenshot 0.png" / "- Screencast 0.mp4" files; embed videos via
  HTML5 <video> blocks
- Re-export Automated school clock v1 README.html with the current
  Org HTML exporter
- Include *.mp4 in the Tools/Update web site rsync filter
- Document the CUSTOM_ID convention in AGENTS.org