Detect existing filters

Lift the logic from the A/B Street version. Be careful with:
- importing non-driveable roads
- representing existing vs new filters
- recording that we deleted an existing filter

Look at the A/B Street version for the filter detection logic. Understand how non-driveable roads are handled and the representation of existing vs new filters. The code likely involves map import and filter management modules. Start by examining the relevant modules in the A/B Street repository to adapt the logic, then integrate it into the LTN codebase, ensuring proper handling of filter deletion recording.

Written by the indexing model from the issue text.