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_review_map.py

@@ -29,7 +29,7 @@ def ensure_writable(path):
     # A packed .umap left read-only (git sets 644->444 on checkout, or the
     # editor re-flags it) makes save_map silently return False with no error.
     # chmod u+w it so the staged camera list and actors actually persist.
-    p = os.path.join(unreal.Paths.project_dir(), path.replace("/Game/", "Content/"))
+    p = os.path.join(unreal.Paths.project_dir(), path.replace("/Game/", "Content/") + ".umap")
     try:
         mode = os.stat(p).st_mode & 0o777
         if mode & 0o200 == 0:
@@ -229,7 +229,7 @@ try:
             # the colonnade's underside.
             ("demo-spawn-view", unreal.Vector(-10722.5, -3500, 165), unreal.Vector(-10722.5, -1450, 700)),
             ("demo-street-level", unreal.Vector(-11000, -400, 165), unreal.Vector(-10722.5, -1450, 500)),
-            ("demo-crossing", unreal.Vector(-13600, -1650, 165), unreal.Vector(-12722.5, -1450, 500)),
+            ("demo-crossing", unreal.Vector(-23750, -1800, 480), unreal.Vector(-24291, -1400, 300)),
             # the two north-side background buildings (TCL, Tussaud's) seen from
             # the south sidewalk, across Hollywood Blvd: their frontage and mass.
             ("demo-north-buildings", unreal.Vector(-22800, -1000, 400), unreal.Vector(-22800, 3000, 300)),