title: Trace a wrong total to its cause in a shared helper
check: python -m pytest -q .bench_check
plan: |
  Milestone 1: fix the wrong totals on multi-currency invoices.

  1. Run `python -m pytest -q`. `test_multi_currency_total` in
     `tests/test_invoice.py` fails: totals are slightly off when an invoice has
     lines in another currency. The test is correct. Do not change any file
     under `tests/`.
  2. Find the cause. Start at `Invoice.total` in `invoicing/invoice.py` and
     follow the functions it calls, reading each one, until you find where
     precision is lost. The cause is in a function that more than one module
     uses.
  3. Fix it at its source, in that shared function or the value it uses. Do
     not special-case currencies in `invoicing/invoice.py` or
     `invoicing/currency.py`: every caller of the shared function has the
     same problem, and all of them must be fixed by your change. The rates in
     `invoicing/currency.py` have up to six decimal places (JPY is 0.006712),
     and no step may round a rate or fraction to fewer places than that.
  4. Run `python -m pytest -q` until every test passes.
  5. Call finish_milestone, with final yes.