Routing supports dead-end u-turns

(copied from https://github.com/a-b-street/ltn/pull/119#discussion_r1954042416)

>I do not have any counter-examples and will have to think about how to force one, but this rule looks like it prohibits U-turns at a deadend. I don't think a route would ever choose to go down one, unless maybe there's a situation where it's forced to by a TR... ie:
>
>```
> D
> |
> A--*--B
>```
>
> If the route starts from B, goes to *, there's a TR preventing B->A, then the route goes to D. It needs to u-turn there to make progress. Then southbound, D->A.
>
> I don't think something this weird is likely to happen in practice, so don't block this PR on it. I've made a note to try and construct this test case later and see if it can actually occur; can file a followup issue if you agree it's possible