gameplay: designated street crossing — zebra markings, corridor cut, up-facing street

- street_mesh.py: continental zebra markings (0.5 m bars, 0.5 m gaps, 3 m
  band) at all three signalised crossings, triangles not quads (FBX SDK
  refuses to triangulate the crown-twisted quads), stripes +6 cm above the
  DEM so they stay above the road surface; winding flipped so every street
  surface faces up (road/sidewalks/markings were 100% down-facing and
  invisible in engine once the terrain stopped covering them).
- terrain_from_dem.py: cut a 10.7 m corridor around the Hollywood Blvd
  centreline so road, kerbs and sidewalks are not buried under the terrain;
  sidewalks themselves cover the transition band, walk-check stays at 0 holes.
- Blender builders save with a writability guard + fatal failure: *.blend
  is LFS-lockable and checks out read-only; a failed save previously left
  downstream steps building from stale files.
- ue_import_site.py: Crossing_Markings imports with no collision (paint on
  the road); ue_build_demo_map.py stages it and chmods the lockable .umap
  before saving (save_map silently returned False on read-only maps).
- ue_build_review_map.py: demo-crossing camera moved to the real crossing
  (s=130.1 m -> UE -24291,-1340) and fixed the ensure_writable path (was
  missing the .umap suffix, so the chmod never fired).

Tools/ue_import_site.py

@@ -27,7 +27,10 @@ def log(msg):
     unreal.log(str(msg))
 
 
-def import_fbx(path, name, collision):
+def import_fbx(path, name, collision="complex"):
+    # 5.8 note: there is no FBX import 'no collision' switch exposed to Python;
+    # a default import simply generates no collision primitives, so collision="none"
+    # means: skip the trace-flag step below and leave the body setup empty.
     task = unreal.AssetImportTask()
     task.filename = path
     task.destination_path = DEST
@@ -58,10 +61,16 @@ def import_fbx(path, name, collision):
     # something to stand on. Terrain and street get complex-as-simple: a simple
     # hull would ramp straight over the kerbs.
     #
-    # UE 5.8 exposes this only through StaticMeshEditorSubsystem (the mesh itself
+    # UE 5.8 exposes this only through the body setup's trace flag (the mesh itself
     # has no trace-flag property, and the subsystem has no setter in Python), so
-    # the import path sets the body setup's trace flag through the mesh's
-    # collision options at import, then verifies it by reading it back.
+    # the import path sets the body setup's trace flag, then verifies it by reading
+    # it back.
+    if collision == "none":
+        box = mesh.get_bounding_box()
+        mn, mx = box.min, box.max
+        log(f"{name:18} import ok  size=({mx.x - mn.x:.0f}, {mx.y - mn.y:.0f}, {mx.z - mn.z:.0f}) cm  "
+            f"min_z={mn.z:.1f}  collision=none")
+        return
     trace = (unreal.CollisionTraceFlag.CTF_USE_COMPLEX_AS_SIMPLE
              if collision == "complex" else unreal.CollisionTraceFlag.CTF_USE_SIMPLE_AND_COMPLEX)
     try:
@@ -83,6 +92,11 @@ def main():
         return
     for path in files:
         stem = os.path.splitext(os.path.basename(path))[0]
+        # Crossing markings are paint on the road surface — no collision at all,
+        # so the player never stands on a 1 cm lip at the band edges.
+        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"
         import_fbx(path, stem, collision)