Earliest-arrival journey planner over a timetable (minimum change times, directed footpaths never chained, trip limit, after-midnight times)

# Earliest-arrival journey planner with change times and footpaths `transit.py` plans trips over our city's offline timetable (`stops.csv`, `timetable.csv`, `footpaths.csv` in a directory). The draft only finds direct trips. Rewrite it (Python 3 stdlib only) so that `Network(directory).query(src, dst, start, max_trips)` follows these rules exactly. ## Data 1. `stops.csv`: `stop,min_change` (seconds). `timetable.csv`: `trip_id,seq,stop,arr,dep`; a trip's stops are visited in increasing `seq` (rows may be in any order). Times are `HH:MM:SS` and hours may be 24 or more (after midnight of the same service day). `footpaths.csv`: `from,to,seconds`, **directed**; if a pair appears several times the smallest time counts; rows with `from == to` are ignored. ## Journeys 2. You are at `src` at time `start` (`HH:MM:SS`). You may **board** a trip at a stop at that row's `dep` if you are ready there at or before `dep`, ride it, and **alight** at any later stop of that trip at its `arr`. 3. **Ready times:** at `src` you are ready at `start`. After alighting at stop s at time a you are ready to board **another** trip at s at `a + min_change[s]`. After walking a footpath you are ready at the arrival time with no change time. 4. **Footpaths** may be walked from `src` before the first trip, after alighting from any trip, and to reach `dst` at the end, but **never two footpaths in a row** (no walking from a stop you reached on foot). 5. `max_trips` (integer >= 0) is the maximum number of trips boarded; with 0 only `src == dst` or a single footpath `src -> dst` works. 6. Reaching `dst` means alighting there or walking a footpath into it (no change time is added at the end). If `src == dst` the answer is `(start, 0)`. 7. **Answer:** `(arrival, trips)` with the earliest possible arrival at `dst` formatted `HH:MM:SS` (hours zero-padded to at least 2 digits, may be >= 24), and among journeys with that arrival the smallest number of trips; `None` if `dst` cannot be reached. An unknown stop name (not in `stops.csv`) raises `KeyError`. 8. **Speed:** on the network in this directory, building the `Network` and answering 400 random queries with `max_trips` up to 4 must take under 20 seconds in total on our shared test box. ## Scoring Hidden check groups (partial credit = fraction passed; all must pass), all on exact answers: `direct_and_parsing` (1-2, 7 on a small hand-made network), `change_times` (3), `footpath_rules` (3-4), `max_trips_and_ties` (5, 7), `after_midnight` (1, 7), `unreachable_and_errors` (6-7), `city_queries` (150 queries on this directory's network), `fresh_city_queries` (150 queries on a freshly generated network of the same kind), `speed` (8).