Consider calendar / calendar dates

Since not all trips are available on all days, we should make sure to only use the available ones. This includes handling calendar.txt/calendar_dates.txt.

In a first step, we should make the algorithm aware of weekdays (calendar.txt).

In a second step, we can add calendar_dates to the mix, which disables some trips on specific dates and adds other trips.

Start by tracing how GTFS trips are loaded and selected, then inspect the handling of calendar.txt and calendar_dates.txt. First verify weekday-based availability; then account for date-specific cancellations and additions, with routing results serving as the completion check.

Written by the indexing model from the issue text.