lighting: fixed daytime rig, real surface materials, crossing fix

Fixed daytime lighting and basic ambience for the DemoMap:

- Sun pitch -50 / yaw 120 (intensity 4, warm), sky light 2.6 with real-time
  capture, sky atmosphere, height fog, and an unbound post-process volume with
  fixed exposure (min = max brightness) so no shot washes out or crushes to
  black depending on how much sky it frames.
- Root cause of the washed-out look was NOT the sun: Blender's flat diffuse
  colours do not survive the FBX->Unreal import. Every site material arrived
  white and the terrain arrived with WorldGridMaterial. The demo builder now
  creates and assigns M_Asphalt / M_Concrete / M_Ground / M_RoadPaint.
- Crossing fix: each zebra band was its own object, so Blender renamed the
  duplicates .001/.002 and site assembly (exact-name append) kept only one.
  All bands are now a single Crossing_Markings mesh (36 verts, 3 bands).
- Terrain gets a Ground_Earth material in the builder so it stops importing
  as a white grid.
- Demo cameras: street-level no longer sits inside the facade body (was
  rendering its back face), and the crossing camera aims at the designated
  s=130.1 crossing (UE X -14956) looking along the crosswalk.

Screenshots re-rendered; the four demo views are marked needs-review for the
owner's approval. MEMORY.md records the material-import, exposure,
light_color, band-naming and camera-bounds traps.

Art/blender/terrain/terrain_from_dem.py

@@ -224,6 +224,12 @@ def main():
     mesh = bpy.data.meshes.new("Terrain_DEM")
     mesh.from_pydata(verts, [], faces)
     mesh.update()
+    # Without a material the mesh imports into Unreal with the default white
+    # material and blows out under the demo sun. A muted earth tone keeps the
+    # ground reading as ground.
+    ground = bpy.data.materials.new("Ground_Earth")
+    ground.diffuse_color = (0.30, 0.28, 0.25, 1.0)
+    mesh.materials.append(ground)
     obj = bpy.data.objects.new("Terrain_DEM", mesh)
     bpy.context.scene.collection.objects.link(obj)