Hey Tim — I've been working on a NIP draft for private on-chain zaps using Silent Payments and your OpenETR work is directly relevant.
Quick technical question before I open the PR: your diagram shows BIP32 derivation for nostr-sp/scan and nostr-sp/spend — is that what your code actually uses, or tagged hashes under the hood? The answer affects interoperability between OpenETR and the NIP.
Happy to share the draft if useful.
