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/terrain/terrain_from_dem.py

@@ -53,6 +53,9 @@ def parse_args():
     ap.add_argument("--ymax", type=float, default=110.0)
     ap.add_argument("--osm", default=None,
                     help="OSM extract; building outlines are cut out so rooftops never become terrain")
+    ap.add_argument("--corridor", type=float, default=10.7,
+                    help="half-width in m of the Hollywood Blvd corridor to cut out so the "
+                         "street meshes (road, kerbs, sidewalks) are not buried under the terrain")
     return ap.parse_args(argv)
 
 
@@ -110,6 +113,30 @@ def pts_flat(ring):
     return ring
 
 
+def centreline_polyline(osm_path):
+    """Hollywood Blvd centreline as a local-frame polyline (Highland -> Orange)."""
+    nodes, ways = oc.load(osm_path)
+    line, _, _, _, _ = oc.block_extent(
+        nodes, ways, "Hollywood Boulevard", "North Orange Drive", "North Highland Avenue")
+    utm = project_to_utm(line, find_proj())
+    return [(e - ORIGIN_E, n - ORIGIN_N) for e, n in utm]
+
+
+def dist_to_polyline(x, y, pts):
+    best = float("inf")
+    for (ax, ay), (bx, by) in zip(pts, pts[1:]):
+        dx, dy = bx - ax, by - ay
+        l2 = dx * dx + dy * dy
+        if l2 == 0.0:
+            continue
+        t = max(0.0, min(1.0, ((x - ax) * dx + (y - ay) * dy) / l2))
+        px, py = ax + t * dx, ay + t * dy
+        d = (x - px) ** 2 + (y - py) ** 2
+        if d < best:
+            best = d
+    return best ** 0.5
+
+
 def inside(x, y, ring):
     """Ray casting point-in-polygon."""
     hit = False
@@ -146,9 +173,12 @@ def main():
           f"X {args.xmin}..{args.xmax}, Y {args.ymin}..{args.ymax}")
 
     polys = []
+    corridor = None
     if args.osm:
         polys = building_polygons(args.osm)
         print(f"masking {len(polys)} building footprints")
+        corridor = centreline_polyline(args.osm)
+        print(f"cutting {args.corridor} m corridor around the Hollywood Blvd centreline")
 
     verts = []
     ok = []
@@ -168,6 +198,10 @@ def main():
                 verts.append((x, y, 0.0))
                 ok.append(False)
                 continue
+            if corridor is not None and dist_to_polyline(x, y, corridor) <= args.corridor:
+                verts.append((x, y, 0.0))
+                ok.append(False)
+                continue
             z = values[ri * width + ci]
             if nodata is not None and z == nodata:
                 verts.append((x, y, 0.0))
@@ -212,7 +246,14 @@ 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}")