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_build_demo_map.py

@@ -500,6 +500,13 @@ def build():
                                roughness=0.72, specular=0.22, noise_scale=0.02,
                                roughness_variation=0.12),
     }
+    # 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)] + \
+               [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",
@@ -514,6 +521,11 @@ def build():
     for mat_name, spec in MATERIAL_SPECS.items():
         built[mat_name] = ensure_material(mat_name, spec, textures)
 
+    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")
+
     imported = 0
     for name, mat_name in SURFACE_MATERIALS.items():
         mesh = unreal.EditorAssetLibrary.load_asset(f"/Game/Hollywood/Site/{name}")
@@ -681,29 +693,10 @@ def build():
     # pavement top, so the capsule started inside the sidewalk block and the downward trace
     # exited through it to Road_Hollywood at 366 - which is what made the character look like
     # it floated while standing on a surface 14 cm above the road.
-    player_z = 469.0
-    # Stopgap collision for the north sidewalk. The staged mesh (Demo_Sidewalk_North, top at
-    # Z 380) does not block a downward trace - the capsule falls straight through it to
-    # Road_Hollywood at 366 - so an invisible cube supplies the surface the player walks on.
-    # The real fix belongs in the kit mesh's collision; this keeps the demo walkable meanwhile.
-    sidewalk_collision = spawn(unreal.StaticMeshActor,
-                               unreal.Vector(-14000.0, -1482.5, 376.0),
-                               "Demo_SidewalkCollision")
-    if sidewalk_collision:
-        cube = unreal.EditorAssetLibrary.load_asset("/Engine/BasicShapes/Cube.Cube")
-        comp = sidewalk_collision.get_component_by_class(unreal.StaticMeshComponent)
-        if cube and comp:
-            comp.set_static_mesh(cube)
-            comp.set_mobility(unreal.ComponentMobility.STATIC)
-            # A 100 cm engine cube: 279 m long, 8.6 m wide, 8 cm thick -> top at Z 380.
-            sidewalk_collision.set_actor_scale3d(unreal.Vector(279.0, 8.6, 0.08))
-            comp.set_collision_profile_name("BlockAll")
-            sidewalk_collision.set_actor_hidden_in_game(True)
-            loc = sidewalk_collision.get_actor_location()
-            scale = sidewalk_collision.get_actor_scale3d()
-            log(f"staged: Demo_SidewalkCollision at Z {loc.z:.0f}, scale Z {scale.z:.3f} "
-                f"(top {loc.z + 50.0 * scale.z:.0f} cm)")
-
+    player_z = 400.0
+    # The invisible sidewalk collision slab is retired: the site export now slices the
+    # sidewalks into 20 m pieces, so the walkable surface is real geometry with an honest
+    # local height instead of a 279 m slab whose bounding box was the highest point anywhere.
     spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1150.0, player_z), "Demo_PlayerStart",
           unreal.Rotator(pitch=0.0, yaw=90.0, roll=0.0))
     log("staged: Demo_PlayerStart on the cut north sidewalk, facing south (+Y)")

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)