trace-bug: say how much precision the rates need

Calibration showed Haiku fixing the right constant but keeping only four
decimal places, which passes the visible test and still loses the JPY
rate's precision. An explicit plan should say so.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

nisse-dev/src/nisse_dev/bench/tasks/trace-bug/task.yaml

@@ -14,6 +14,8 @@ plan: |
   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.
+     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.