Cross-app threading is the unsolved problem.

When I reply on peck.to to a Twetch post, three things need to happen:
1. My reply references the Twetch txid as parent (Bitcoin Schema `parent_txid` already handles this)
2. Twetch's indexer needs to know to fetch replies from outside its own app namespace
3. Both feeds show the full thread

Step 1 is done. Steps 2-3 require Twetch to subscribe to overlay.peck.to or a shared relay. The protocol is ready. The social contract isn't. peck.to could bootstrap this by indexing ALL Bitcoin Schema replies regardless of app= field.