Nothing checks that links inside a doc resolve — only reachability from docs/README.md

[pal-tamas]

DocsIndexTests guards that every doc under docs/ is reachable from docs/README.md, and GuidesTests guards GuideCatalog parity both ways. Neither checks that a link written inside a doc points at something that exists.

Two real misses, both caught by a person reading rather than by a test

1. A dead anchor. docs/sqlite.md linked [Where OPFS fits](#rasksqlitebrowser--keeping-a-browser-database) while "Where OPFS fits" was bold text, not a heading — so it had no anchor and the link silently landed the reader at the top of the section. Nothing looked broken, which is the worst kind of broken link.
2. A link to a file that does not exist yet. docs/sync.md (feat(sync): add Rask.Sync, the merge engine offline-first sync rests on #654) cross-linked object-storage.md, which lives in the still-unmerged feat(objectstore): add Rask.ObjectStore, an S3 and Azure Blob client with no cloud SDK #653. Had the merge order gone the other way it would have shipped a 404. I removed the links rather than make the doc depend on merge order, which is a workaround, not a fix — the next person writing a doc across two PRs has no reason to think of it.

Both are the same class: the docs are the product's front door and these are exactly the failures a reader hits first.

What a guard would need to cover

• Relative *.md links resolve to a file that exists. Note these are rewritten at render time to SPA routes (Markdown.RewriteLinks → /guides/{leaf}), so the check belongs on the source text, and a link to a doc that exists but is not in GuideCatalog renders as a route that 404s — worth failing on too.
• In-page anchors (#…) resolve to a heading in the target document, including cross-document other.md#section. This is the one that caught nobody, because a wrong anchor still navigates.
• Ignore external http(s) links: those need the network and would make the suite flaky for no benefit.

Where it belongs

Alongside DocsIndexTests in Rask.Example.Shared.Tests, which already enumerates docs/**/*.md on disk and already owns the reachability half. It is the same walk with a different assertion.

One interaction worth knowing: until the docs/ entry lands in the pre-commit filter (#652), a docs-only commit skips these tests entirely — so this guard and that filter fix are worth having together, or the new check will not run for precisely the commits that most need it.