Simplify and handle failures in spawning

I constantly lose track of the convoluted flow of spawning, from the soundcast representation to `SpawnTrip` to `TripSpec` to the Trips manager. Things like biking connections can fail, and ideally that'd just result in an aborted trip. But right now, there are sometimes fallbacks to walking, sometimes just panics.

When do I need to regenerate scenarios? Does any of that indirectly ask about `biking_connection`?

Aside: `biking_connection` doesn't factor in the start/endpoint. Sometimes two things within a disconnected pocket are reachable. Deal with that, or just solve the disconnectedness?