Rover: Dijkastras should handle case where vehicle is started outside of fence

Rover can end up outside the fence without the user doing anything particularly bad:

- create a polygon fence
- set LOIT_RADIUS to 10m
- place a waypoint with a 60s delay closer than 10m to the fence boundary

When the rover loiters at the waypoint it may stray outside the fence, RTL will trigger but the vehicle will not be able to return

To resolve this we should enhance Dijkstras so that it handles the case where the vehicle starts outside the fence

Start by locating the Dijkstras implementation and the RTL fence-handling entry point. Reproduce the described rover case with a polygon fence, LOIT_RADIUS of 10m, and a delayed waypoint near the boundary, then trace how routing behaves when the vehicle starts outside the fence. Done means RTL can produce a valid return path in that situation.

Written by the indexing model from the issue text.