Support gravel surfaces in OSM road imports

[vanlueckn]

Summary

• Import OSM field tracks as gravel unless tags indicate a paved surface.
• Add surface and tracktype mapping for asphalt, concrete, and gravel roads.
• Render gravel roads with dedicated textures and higher roughness.
• Expose gravel in the route editor, localization strings, documentation, and example module.

Testing

• Added unit coverage for OSM surface and tracktype handling.
• Not run locally.