Infinite parking

Some leftover issues:

- [X] Why's the efficiency layer still showing some people parked far away?
- Fixed: zero-length walking paths are handled oddly by the pathfinding layer
- Some people will use a car on a later trip, so it's staged at some other house
- [x] Properly handle blackholes, nobody should ever fail
- [X] Why is the parking overhead tab showing people walking far?
- The parking phase starts at the last lane, so driving time along it is captured
- [ ] Cleanup implementation: probably consolidate the two impls