Bill a month of electricity from shuffled meter reads with estimates, rollovers and a meter exchange

# Bill September electricity from meter reads Inputs: `meters.csv` (each account's meters, including one exchange), `reads.csv` (actual "A" and estimated "E" reads, in no order) and `tariff.json`. Bill every account for the period from the opening read date to the closing read date in `tariff.json`. - Consumption = closing read minus opening read for each account. - When a meter has both an estimated and an actual read on the same date, use the actual read. An estimated read with no actual read for that date stands. - Meters have 5 dials and roll over from 99999 to 00000. A closing read below the opening read means the meter rolled over once. - If a meter was exchanged during the period, consumption = (old meter's final reading minus its opening read) + (new meter's closing read minus its initial reading). - Energy charge: the first `tier1_kwh` kWh at `tier1_rate`, the rest at `tier2_rate`. - Bill = energy charge + standing charge for 30 days, plus VAT on the whole. - Round each bill to cents half-up only at the end; the grand total is the sum of the rounded bills. Write `out/bills.json`: `{"kwh": {"<account_id>": <kWh>}, "bill_total": {"<account_id>": <amount>}, "grand_total": <amount>}`. ## How this run works - Work on your own: nobody will answer questions in this task, and writing to `ask/` counts against the run. - Write only under `out/`. Create `out/` if it is missing. Inputs are synthetic; don't fetch anything from the network. - Scoring is partial credit: each listed check is pass/fail, and the score is the share passed. Numbers are compared to within 0.005 unless the task says otherwise.