Hourly kWh from raw cumulative meter readings (mixed time zones, duplicates, counter rollover, resets, >6 h gaps, linear interpolation)

# Task: hourly energy consumption from raw meter readings `readings.csv` holds raw cumulative kWh counter readings from several electricity meters (`meter_id,ts,reading`). Write `hourly.py` (Python stdlib only) that reads `readings.csv` from the current directory and writes `hourly.csv` there, following the rules below. Run it. The grader compares `hourly.csv` with the correct result, then re-runs `hourly.py` on a freshly generated file of the same kind, so do not hard-code anything. No network. ## Cleaning 1. `ts` is ISO 8601 and may end in `Z`, in an offset such as `+02:00` or `-05:00`, or have no offset (then it is UTC). The separator between date and time may be `T` or a space. Convert everything to UTC. 2. `reading` is a decimal number; some rows use a comma as the decimal separator (`1234,567`). 3. Rows are not sorted. If a meter has several rows for the same instant (after converting to UTC), keep only the LAST of them in file order. ## Consumption between consecutive readings (per meter, in time order) 4. Normally consumption = later reading - earlier reading. 5. Rollover: the counter wraps from 99999.999 to 0. If the reading drops by more than 90000, it rolled over: consumption = later + 100000 - earlier. 6. Any other drop means the meter was reset or replaced: consumption in that interval is unknown. 7. If two consecutive readings are more than 6 hours apart (strictly; exactly 6 hours is fine), consumption in that interval is unknown. ## Hourly output 8. Between consecutive readings, consumption is spread linearly over time (so the cumulative usage curve is piecewise linear through the readings). 9. For each meter, output one row per UTC hour slot `[H, H+1h)` with H on a whole hour, for every slot that lies completely between the meter's first and last reading (H >= first reading time and H+1h <= last reading time). 10. `kwh` for a slot = cumulative usage at H+1h minus cumulative usage at H, using the linear curve of rule 8. If the slot overlaps an unknown interval (rules 6-7) for any positive length of time, `kwh` is empty. 11. `hourly.csv` header `meter_id,hour_start,kwh`; `hour_start` formatted `YYYY-MM-DDTHH:00:00Z`; `kwh` with 3 decimals (or empty); rows sorted by `meter_id`, then `hour_start`. Grading: the set of (meter_id, hour_start) rows must match exactly, empty `kwh` must be empty exactly where expected, and every other `kwh` must be within 0.002 of the correct value. Same on the fresh dataset.