Walking through a potential on chain Bitcoin implementation: 1. Sender taps zap button on a recipient's note. 2. Sender's nostr client generates a bitcoin URI that also includes a flag that it's a nostr zap payemnt. 3. Sender opens the URI with a Bitcoin wallet app, which auto populates the payment fields. 4. After successful send, the Bitcoin wallet generates a nostr URI that has the transaction info filled in. 5. Sender opens that URI with a nostr client, which then sends an event containing the original note id and transaction hash and expected recipient address to a relay. 6. Relay checks the Bitcoin chain for the transaction and if valid, updates the zap counter for the note. In step 5, the sender can be prompted to manually enter the transaction info so the Bitcoin wallet doesn't need to generate a nostr URI. But a nostr URI would be nice and simple. Also this flow is obviously easily interchangeable with Monero or Ethereum.