Integrating jhalfs Source Metadata

Goal: reuse jhalfs wget-list and md5sums to populate package source.urls and auto-fill checksums when harvesting metadata for MLFS/BLFS/GLFS packages.
Data source: https://anduin.linuxfromscratch.org/ hosts per-release wget-list/md5sums files already curated by the jhalfs project.
Approach:
1. Fetch (and optionally cache under ai/cache/) the lists for each book.
2. When harvesting, map <package>-<version> against the list to gather all relevant URLs.
3. Pull matching checksum entries to populate source.checksums.
4. Keep the existing HTML scrape for chapter/stage text; jhalfs covers only sources.
Benefits: avoids fragile HTML tables, keeps URLs aligned with official build scripts, and ensures checksums are up-to-date.