BRouter supports a `polygons` parameter with full vertex coordinates.
We were approximating polygons as circles (centroid + max radius),
which poorly represented elongated or concave shapes.
Now passes polygon vertices directly — no approximation, exact
no-go boundaries.
Also removes unused haversineMeters helper and updates the spec.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>