fix: purge dead offprint links and filter .test domains
- delete 31 stale docs from turso (16 dead offprint links, 18 .test domains, 3 overlap)
- add .test domain filter in tap.zig (processPublication) and indexer.zig (insertDocument)
- revert offprint URL slug construction — old docs purged, only /a/ format remains
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>