fix(site): slice the road too - the 2 m lid was a convex hull, not geometry

The road was never mis-modelled: a 279 m strip imports with ONE auto-generated convex hull, and a
convex hull's top is the strip's maximum height. A downward trace therefore landed at 366 cm - a flat
lid about 2 m above the visible road, which sits at roughly 152 cm there. That single fact explains
every failed sidewalk fix, the 'road above its own sidewalk' reading, and why the player always ended
up on Road_Hollywood.

Slicing the road the way the sidewalks already were fixes it, and the cross-section is now continuous:

  Terrain_DEM       154
  Sidewalk_North_07 158
  Road_Hollywood_07 158   (was 366)

Two changes had to land together: the road exported in slices, and the map stopped staging the single
Road_Hollywood asset - which still sat on top of the slices and won every trace until removed.

All 23 captures regenerated; the player and the car both stand on a continuous ground plane.

Tools/ue_build_demo_map.py

@@ -503,17 +503,13 @@ def build():
     # Segmented ground: the site export slices sidewalks and kerbs into 20 m pieces, so the
     # map stages every slice. A single 279 m slab could not report an honest local height and
     # its collision was degenerate; slices fix both.
-    segments = [f"Sidewalk_North_{i:02d}" for i in range(14)] + \
+    segments = [f"Road_Hollywood_{i:02d}" for i in range(14)] + \
+               [f"Sidewalk_North_{i:02d}" for i in range(14)] + \
                [f"Sidewalk_South_{i:02d}" for i in range(14)] + \
                [f"Kerb_North_{i:02d}" for i in range(14)] + \
                [f"Kerb_South_{i:02d}" for i in range(14)]
     SURFACE_MATERIALS = {
         "Terrain_DEM": "M_Ground",
-        "Road_Hollywood": "M_Asphalt",
-        "Sidewalk_South": "M_Concrete",
-        "Sidewalk_North": "M_Concrete",
-        "Kerb_South": "M_KerbConcrete",
-        "Kerb_North": "M_KerbConcrete",
         "Crossing_Markings": "M_RoadPaint",
     }
     textures = ensure_textures()
@@ -523,8 +519,12 @@ def build():
 
     for segment in segments:
         if unreal.EditorAssetLibrary.does_asset_exist(f"/Game/Hollywood/Site/{segment}"):
-            SURFACE_MATERIALS[segment] = ("M_Concrete" if "Sidewalk" in segment
-                                          else "M_KerbConcrete")
+            if "Road" in segment:
+                SURFACE_MATERIALS[segment] = "M_Asphalt"
+            elif "Sidewalk" in segment:
+                SURFACE_MATERIALS[segment] = "M_Concrete"
+            else:
+                SURFACE_MATERIALS[segment] = "M_KerbConcrete"
 
     imported = 0
     for name, mat_name in SURFACE_MATERIALS.items():

Tools/ue_import_site.py

@@ -109,8 +109,8 @@ def main():
             continue
         # Segmented ground surfaces keep their names with a numeric suffix; both the base
         # names and the slices want the same walkable collision.
-        walkable = ("Terrain_DEM", "Road_Hollywood", "Sidewalk_South", "Sidewalk_North",
-                    "Kerb_South", "Kerb_North")
+        walkable = ("Terrain_DEM", "Road_Hollywood", "Road_Hollywood_",
+                    "Sidewalk_South", "Sidewalk_North", "Kerb_South", "Kerb_North")
         collision = "complex" if any(stem.startswith(w) for w in walkable) else "simple"
         import_fbx(path, stem, collision)
     unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)