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.

Tools/ue_build_review_map.py

@@ -258,8 +258,17 @@ try:
             # 14 m back so the 14.5 m elevation fits: at 2 m the camera sees only
             # 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(-23750, -1800, 480), unreal.Vector(-24291, -1400, 300)),
+            # Street level, from the north sidewalk: the old camera sat at Y -400,
+            # inside the facade body (which spans Y -1457..-200), so it looked at
+            # the unlit back face. Y -2600 is on the sidewalk in front of the lit
+            # elevation.
+            ("demo-street-level", unreal.Vector(-10850, -2600, 170), unreal.Vector(-10722.5, -1450, 400)),
+            # The designated mid-block crossing. Street arc length runs from the
+            # west end (s=0 at X -27966 cm), so s=130.1 m is UE X -14956, not the
+            # -24291 the old camera used (that is the s=36.6 crossing). Camera sits
+            # on the north sidewalk looking along the crosswalk itself, which is
+            # the pedestrian's view and shows the zebra bars as bars.
+            ("demo-crossing", unreal.Vector(-15250, -2500, 400), unreal.Vector(-14956, 400, 150)),
             # 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)),