Checkpoint: Architecture

Checkpoint — an injected checkout

Implement CheckoutService(repo, clock):

• checkout(cart) builds {'id': f'o-{clock.now():.0f}', 'items': list(cart), 'total': sum(cart)}, saves it via the repository, and returns it
• an empty cart raises ValueError('empty cart') — nothing saved
• the clock is INJECTED (clock.now() returns a float) — the graded test uses a fixed fake clock and asserts the id and totals exactly

Constructor injection + injected time = deterministic tests. That is the lesson.