Record a Rillet customer payment against the right invoice

**Objective:** Record a customer payment of `{amount}` against invoice `{invoice-number}`, dated `{payment-date}` (default today), into the cash account `{account-code}` (ask if not given).

**Step 1: Find the invoice**

Call `rillet-get-invoices` (it takes no filters) and find the invoice whose `invoice-number` is exactly `{invoice-number}`. If none or more than one match, say so and stop. Call `rillet-get-customers` to name its customer.

**Step 2: Check what is owed**

* Call `rillet-get-invoice-payments` (no filters) and list the payments already recorded against this `invoice-id`.
* Compare `{amount}` with `amount-due`. If `{amount}` is more than `amount-due`, stop and say so; do not record an overpayment.
* The payment currency is the invoice's `currency`.

**Step 3: Confirm the cash account**

If `{account-code}` is empty, call `rillet-get-accounts` and offer the active asset accounts that look like bank or cash accounts. Either way, confirm the account exists and is active.

**Step 4: Confirm, then write once**

Show the user: customer, invoice number, invoice total, amount due, the payment amount, date, currency and account, and the amount due after the payment. Ask them to confirm. Only after they say yes, call `rillet-add-invoice-payment` once, with `invoice-id`, `amount`, `currency`, `date` and `account-code`.

**Step 5: Verify**

Call `rillet-get-invoice-payments` again and show the new payment, then call `rillet-get-invoices` and show the new `amount-due`.

**Rules & Guidelines:**

* `rillet-add-invoice-payment` is the only write tool you may call, and only once, after confirmation.
* Never call `rillet-update-invoice-payment` to change an existing payment, and never call `rillet-update-invoice`.
* If the call fails, report the error and do not retry with changed values unless the user asks.

**Validation:**

* If `{amount}` is not a positive number, ask again.
* If the invoice is already fully paid, say so and stop.