Settle invoices through the MorokInvoices contract so a till can confirm a sale from chain state instead of a private balance guess.

The QR now carries a Poseidon commitment over a per-merchant secret and the invoice number. The pool forwards it to privacy_invoke, which logs InvoiceSettled; the sell page watches that event over RPC. Only the hash is public.

Co-authored-by: Cursor <cursoragent@cursor.com>