Build the plan to pay an invoice

method "transfer" when tokenIn is the invoice token: a direct transfer built from the stored invoice, as a one-step plan (EVM steps[] with a transaction step; Solana a transaction without executionRef, broadcast via /svm/execute). method "swap" otherwise: amountIn is computed so the guaranteed minimum output covers the requested amount, through the route with GAIN when there is one, delivered to the recipient; plan is a /swap plan (EVM) or a /svm/swap build (Solana). Sign it like any plan, then POST the hash to /invoices/{id}/payments.