c3f25a34

By: Michael Lynch <git@mtlynch.io>

Revalidate external links when runner dependencies change

Previously, the external validation manifest identified lychee, its configuration, and the checking script separately. Changes to the CA trust store or wrapper could reuse an older successful result.

Key validation by the packaged runner instead, including its executable policy and dependencies. Isolate fixture runs from existing store results and exercise the shipped exclusion policy without crawling linked pages. Document Markdown-only external coverage and cold-check costs.

The full offline HTML check also exposed a missing book-cover image repeated across 247 pages. Point the ad at the existing image rather than suppress the failures.