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).

Art/blender/site/site_assembly.py

@@ -63,6 +63,7 @@ APPEND = {
     "Art/blender/terrain/terrain-2026-11-06.blend": ["Terrain_DEM"],
     "Art/blender/street/street-2026-11-06.blend": [
         "Road_Hollywood", "Sidewalk_South", "Sidewalk_North", "Kerb_South", "Kerb_North",
+        "Crossing_Markings",
     ],
 }
 BLOCKOUT = "Art/blender/blockout/blockout-2026-11-06.blend"
@@ -193,7 +194,14 @@ def main():
 
     out = os.path.abspath(args.out)
     os.makedirs(os.path.dirname(out), exist_ok=True)
-    bpy.ops.wm.save_as_mainfile(filepath=out)
+    # *.blend is LFS-lockable, so a fresh checkout marks it read-only. Make it
+    # writable again or the save fails and downstream steps build from a stale
+    # file — which is worse than failing here.
+    if os.path.exists(out) and not os.access(out, os.W_OK):
+        os.chmod(out, 0o644)
+    result = bpy.ops.wm.save_as_mainfile(filepath=out)
+    if result != {"FINISHED"}:
+        raise RuntimeError(f"could not save {out}: {result}")
     print(f"saved {out}")