bug: in demo, distance isn't calculated for arcs that cross the dateline

i suspect all that's needed to fix is to guard for MultiLineStrings and sum individual segments.

> Last route distance: NaN km

<img width="874" height="824" alt="Image" src="https://github.com/user-attachments/assets/64c27d61-b0d1-484d-86cf-2d520b699fab" />