Fix the rounding diff check for a billing refactor

An agent refactored your billing module, and the unit tests still pass. A close read shows one change in behavior: the old proration code rounded half a cent up, and the new code rounds half to even (banker's rounding). Before you merge, you run both rounding rules over a large set of invoice rows and list every row where the amounts differ.

An assistant wrote proration_rounding_divergences(monthly_cents, days_used, days_in_period). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• monthly_cents, days_used, days_in_period: one invoice row per index: the monthly price in cents, the days the customer used, and the days in the billing period.
• Return the indices of the rows where the two rules give different amounts, in increasing order.

Rules the tests check:

• If the lists differ in length, use only the indices all three have.
• Skip a row whose days_in_period is 0 or less.
• Treat a negative price as 0. Clamp days_used to the range 0 to days_in_period.
• The exact prorated amount, in cents, is price times days used, divided by days in the period. Work with exact integers, not floating point.
• Half up: round to the nearest whole cent, and an amount exactly halfway between two cents goes up.
• Half to even: round to the nearest whole cent, and an amount exactly halfway goes to whichever of the two neighbors is even.
• A row diverges when the two rules give different whole cents.