CSV-to-SQLite modernization fixture

Read requirements.md. Preserve legacy.py, service.py and the baseline tests; complete migrate.py and order_store.py.

python -m unittest test_legacy -v
python -m unittest test_modernization -v

The first command is a working baseline; the second fails until modernization is implemented. Tests create temporary files under the configured TMPDIR. Set it to the work drive before running.

Instructor validation from this source directory:

HANDS_ON_REFERENCE=1 python -m unittest test_legacy test_modernization -v

The reference must reconcile 3 records and 6249 cents, preserve source bytes, refuse overwrite, reject malformed data, and support CSV rollback. This is a local training migration, not proof of production crash durability or concurrent-writer safety.