Add lane marking overlay, rural ditches, and 15 cm urban curbs

[Copilot]

LaneMarkingTextureId was generated but never applied to any renderer, making lane lines invisible. Rural roads lacked roadside geometry entirely, and urban curbs were only 5 cm tall.

Lane markings

• BuildLaneMarkingMesh() duplicates road surface geometry elevated by 5 mm (LaneMarkingClearance) with UV0 set to lane-marking tiling (6 m repeat) — generated for all paved types (not Dirt/Path/Cycleway)
• MapSceneBuilder.BuildRoad() now creates a LaneMarking child GameObject and applies lane_marking_oneway / lane_marking_twoway material

Rural ditches (1 m deep)

• BuildDitchMesh() generates a V-profile trench on both sides: road edge → bottom (1.5 m out, 1 m below road surface) → outer edge (3 m out, back to road level)
• RegionTextures.GetDitchTextureId() returns region-appropriate texture (terrain_grass, terrain_sand, terrain_snow, terrain_mud)
• MapSceneBuilder creates a Ditch child GameObject for rural roads

Urban curbs (15 cm)

• IsUrbanRoadType() classifies Residential and Service as urban; everything else is rural
• UrbanKerbHeight = 0.15f auto-selected for urban roads via the ExtrudeWithDetails(RoadType, ...) overload; rural roads get kerbHeight = 0 (empty kerb mesh) + ditch

RoadMeshResult additions

public readonly Mesh? LaneMarkingMesh;   // overlay mesh, UV0 = lane-marking tiling
public readonly Mesh? DitchMesh;         // null for urban roads
public readonly string DitchTextureId;   // empty when DitchMesh is null

Original prompt

Make sure lane lines and markers are visible in game - I don't think they currently are for some reason.

Also, rural roads should have 1 meter deep ditches on both sides of the road, particularly if it's a flat area.

urban roads should generally have a 15 cm curb that then smooths to the terrain.

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