Highway preference, without a second router

use_highways on /route and /isochrone is Valhalla's existing auto and truck costing option on the Canada OSM graph. Industry and remote routing stay experimental: no winter-road graph, no resource-road costing.

• use_highways from 0 (avoid motorways and trunks) to 1 (prefer), on auto, car, and truck only
• bicycle and pedestrian reject the flag with a 400 that names the allowed modes
• Docs on /docs/apis/routing and /docs/reference/coverage say what the graph does not model

AvailabilityAvailability was not re-checked against production for this record. use_highways applies to auto, car, and truck. There is no winter-road graph.