Consider transfers.txt

Nobody has claimed this yet.

Right now, we only account for direct switches between connections. We should add transfers to the mix to allow routing on a full network.

Transfers come in guaranteed transfers (transfer time == 0) and minimum transfer_time transfers.
Both can be handled the same by storing all transfers for a stop.

We can probably optimise the routing quite a bit, if we identify all reasonable transfer locations in advance.

A transfer is only reasonable at station `x+1` if we can reach an earlier train at `x+1` that we couldn't reach at `x` or can switch to a line that did not exist at `x`. So if `lines(x) != lines(x+1) || \exists(line) : get_earliest_departure(line,x+1) < get_earliest_departure(line,x)`, we consider transferring. This is only the case if some line overtakes another between `x,x+1`. So transfers are necessary only if a line `a` exists that overtakes line `b` between `x,x+1`.

Otherwise we could have done the transfer at `x`.

This allows for an analysis of all stops for possible transfers. We simply scan all lines that are present at `x` and `x-1` and see if the current line overtakes any other line between `x-1` and `x`. If so, or if `x` contains a new line, we mark `x` as transfer location.

Transfers to include:

- [ ] station to station + same stop transfers, if not already part of transfers.txt, given they fulfil any of the following
- [ ] same line: `previous(this) != next(reverse(this))` (the opposing direction services a stop that we did not reach ourselves)
- [ ] new lines become available
- [ ] lines overtake at this stop (stop a and stop b are serviced by two lines, one line reaches a later but b prior to the other line)
- [ ] ensure that ^ covers all required transfers

Start by inspecting the existing direct-switch routing logic and how transfers.txt is currently represented, if it is present. Compare the implementation with the listed station, same-line, new-line, and overtaking cases; done means all required transfers are included and routing works across the full network.

Written by the indexing model from the issue text.