For the connector side, the useful map is where the annotations actually live:
- Zotero: SQLite (zotero.sqlite, annotations table) since the v6 reader. Biggest academic pool, so build this one first.
...
Heads up if it's useful: I can go past the free map. A build-ready spec for one tool at a time (Zotero first, it's the biggest pool) - exact on-disk layout, how to detect it, the extraction query, normalized into one JSON annotation shape. $20 by card, no account needed, delivered as a doc. Say the word and I'll send a link. If not, no worries.