roads: enable Rugged+Trail carving + per-tier character (D-022)

All four road tiers now carve into the 3D world. Rugged and Trail were
generated, exported and parsed all along, but nothing ever consumed them —
which is why county roads visible on the 2D map did not exist in 3D.

Tier identity is now carried into the carve via a RoadSegment struct, so each
tier gets its own width, shoulder and grade-smoothing from one tunable block in
Constants: highways widest and holding a grade, trails narrow and hugging the
land. Rugged and Trail surface as dirt rather than asphalt.

Because tiers now have different reach, nearest-by-distance was no longer a
sound way to pick the governing road — a nearby footpath could shadow a highway
whose shoulder still covered the column. The carve now considers only roads
whose shoulder actually reaches, and takes the closest of those.

The per-chunk cull pad is derived from the widest shoulder plus a margin, so
widening a road cannot silently truncate it at chunk edges.

Untouched: density field, Marching Cubes interpolation/welding, chunk dims,
the .dat contract, the 2D A* generation, and mesher normals.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Client/Scripts/ChunkRenderer.cs

@@ -12,7 +12,7 @@ namespace IslaApocalypse.Client
 			//    surface/biome/road data it needs to fade colours smoothly.
 			Mesh = MarchingCubes.GenerateMesh(
 				data.Densities, data.BlockIDs,
-				data.SurfaceHeights, data.ColumnBiomes, data.ColumnIsRoad,
+				data.SurfaceHeights, data.ColumnBiomes, data.ColumnRoadMaterial,
 				Constants.ISO_LEVEL, Constants.VOXEL_SCALE);