A wholesale shop gives 20% off any cart line where the customer buys 10 or more of the
same item. cart is a list of [price_cents, quantity] lines. For each line compute
price_cents * quantity; if quantity >= 10, the line costs 80% of that, rounded down
to a whole cent. Return the cart total in cents.
Examples
Input: cart = [[250, 2], [100, 10]]
Output: 1300
Explanation: 500 + (1000 with 20% off = 800) = 1300
Input: cart = [[199, 3]]
Output: 597
Constraints
0 <= len(cart) <= 1000,1 <= price_cents <= 10**5,1 <= quantity <= 1000
Goals
- Compute a per-line total and apply a conditional discount to it
- Use integer arithmetic on cents to avoid rounding surprises