title: Fix a bug a failing test exposes
check: python -m pytest -q .bench_check
plan: |
  Milestone 1: fix the failing inventory test.

  1. Run `python -m pytest -q` and read the failure.
  2. The failing test is correct; the bug is in `inventory.py`. Do not change
     any test file.
  3. Fix `total_value` so that each item contributes its unit price times its
     quantity.
  4. Run `python -m pytest -q` until every test passes.
  5. Call finish_milestone, with final yes.