Challenge 2: Design a src/ Layout — Possible Solution
====================================================================

calculator/
├── pyproject.toml
├── README.md
├── src/
│   └── calculator/
│       ├── __init__.py
│       ├── basic.py
│       └── advanced.py
└── tests/
    ├── test_basic.py
    └── test_advanced.py

WHY THIS WORKS AS AN ANSWER
------------------------------
This reuses this chapter's own src/ layout tree exactly, keeping every
element the original example established: pyproject.toml and
README.md at the project root, an src/calculator/ folder holding the
actual importable package code, and a separate top-level tests/
folder — the same "package code separate from tests and config"
separation the chapter's warn-box-adjacent explanation calls out as
the whole point of this layout.

__init__.py is kept inside src/calculator/, reusing the chapter's own
note that this file is specifically what makes the calculator folder
importable as a real Python package at all — without it, basic.py and
advanced.py would just be loose scripts, not part of an installable
calculator package.

basic.py and advanced.py are added as two separate modules inside the
package folder, directly matching what the challenge specifies —
splitting calculator's code into two files rather than one large
file, the same module-based organization already covered back in
Course 1, Chapter 9's own import/module coverage.

test_basic.py and test_advanced.py mirror the two source modules
one-to-one inside tests/, following the same test_*.py naming
convention pytest auto-discovers (Course 2, Chapter 8) — one test
file per module keeps it clear which tests exercise which piece of
the package.