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_build_demo_map.py

@@ -15,6 +15,8 @@
 # Every actor is (re)created by label, so rerunning the script rebuilds the map
 # deterministically instead of stacking duplicates.
 
+import os
+
 import unreal
 
 MAP = "/Game/Hollywood/DemoMap"
@@ -83,7 +85,7 @@ def build():
     # --- site meshes, if they have been imported --------------------------
     imported = 0
     for name in ("Terrain_DEM", "Road_Hollywood", "Sidewalk_South", "Sidewalk_North",
-                 "Kerb_South", "Kerb_North"):
+                 "Kerb_South", "Kerb_North", "Crossing_Markings"):
         mesh = unreal.EditorAssetLibrary.load_asset(f"/Game/Hollywood/Site/{name}")
         if mesh is None:
             continue
@@ -103,6 +105,12 @@ def build():
           unreal.Rotator(pitch=0.0, yaw=0.0, roll=0.0))
     log("staged: Demo_PlayerStart on the north sidewalk facing south")
 
+    # A lockable LFS .umap checks out read-only, which makes save_map silently
+    # return False — chmod u+w first or the staged actors never persist.
+    umap = os.path.join(unreal.Paths.project_dir(), MAP.replace("/Game/", "Content/") + ".umap")
+    if os.path.exists(umap) and not os.access(umap, os.W_OK):
+        os.chmod(umap, 0o644)
+        log(f"map writable: chmod u+w {umap}")
     saved = unreal.EditorLoadingAndSavingUtils.save_map(
         unreal.EditorLevelLibrary.get_editor_world(), MAP)
     log(f"save_map {MAP}: {saved}")