feat(site): slice the ground into 20 m segments - and the slice hypothesis dies with them

The site exporter now splits sidewalks and kerbs into ~20 m pieces
(SEGMENTED / SEGMENT_M in Art/blender/site/export_site_fbx.py), 56 slices in all, so each has a
bounding box that means something where it stands instead of reporting the highest point on a
279 m slope. They import and stage (63 site meshes in the demo map).

The hypothesis behind it is dead, and that is the useful part: with the invisible sidewalk
stopgap retired, the player now falls to Road_Hollywood at 366 - the slices do not block either.
So the 279 m aspect ratio was never the cause; a 20 m slice of the same mesh fails identically.
The sidewalk geometry itself is what does not collide, and the next place to look is its shape -
an open surface with no volume, inverted winding or zero-thickness triangles - not the import
flags and not the size.

The stopgap retirement stands: the player walks on the road, which is real geometry that does
collide, rather than on an invisible ledge.

Tools/ue_import_site.py

@@ -107,8 +107,11 @@ def main():
         if stem == "Crossing_Markings":
             import_fbx(path, stem, "none")
             continue
-        collision = "complex" if stem in ("Terrain_DEM", "Road_Hollywood",
-                                          "Sidewalk_South", "Sidewalk_North") else "simple"
+        # 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")
+        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)