test(docs): recursive link + heading-anchor integrity check

Broaden the guard from README + top-level docs/*.md inline links to the full
tracked doc set and more link forms:

- scan README.md, CHANGELOG.md, FINDINGS.md, docs/**/*.md, and eval/**/README.md
  (eval run logs are skipped — only their READMEs are prose)
- parse inline links, image links, reference-style definitions, and simple HTML
  href=/src=
- validate the target file exists AND, when a link carries a #fragment into a
  markdown file, that a matching GitHub-style heading anchor exists

Verified: passes on the current tree and fails on an injected broken link or
broken #anchor. (A README public-API-example typecheck lands on the integration
branch, where the README import fix is present.)