Rethink how severance / WithTraffic connections are handled

The preprocessing is brute force. It might mess up the "cross anywhere" strategy. The current approach has some TODOs to be careful with.

It might be worth explicitly modeling all or some turns upstream in `graph`, but at what perf cost?

The issue mentions preprocessing in the graph and a 'cross anywhere' strategy. Look at the code around severance and WithTraffic connections, likely in the graph module. Understand the current brute-force approach and the TODOs. Evaluate performance implications of modeling turns upstream. Start by finding where connections are handled and the existing TODOs.

Written by the indexing model from the issue text.