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.

Screenshots/README.md

@@ -22,8 +22,14 @@ and exits — about a minute. The camera list lives in
 - **Floor**: 160 m square, top face at Z 0. The character measures Z 0..83 cm.
 - **Cubes**: `Cube_ScaleTest_A` rests on the floor, `Cube_ScaleTest_B` floats at
   Z 2 m; both are exactly 1 m — the scale reference.
-- **Lighting**: one warm key light plus sky fill (review rig, not the `0.1.0`
-  daytime look). Beyond the floor is open: the horizon is a black band.
+- **Lighting (review map)**: one warm key light plus sky fill, neutral exposure.
+- **Lighting (demo map)**: fixed daytime — sun at pitch -50 / yaw 120 (intensity
+  4, warm), sky light 2.6, sky atmosphere, height fog, and a manual-exposure
+  post-process volume (min = max brightness 1.0) so a shot cannot wash out or
+  crush to black depending on how much sky it frames. Surface colours are
+  assigned explicitly in `Tools/ue_build_demo_map.py` (`M_Asphalt`, `M_Concrete`,
+  `M_Ground`, `M_RoadPaint`) because Blender's flat diffuse colours do not
+  survive the FBX → Unreal import.
 
 ## Review-map shots
 
@@ -53,10 +59,10 @@ Rendered from `/Game/Hollywood/DemoMap`, the walkable level. Click an image for
 
 | File | Preview | Approval | What it shows |
 | --- | --- | --- | --- |
-| `demo/review-demo-spawn-view.png` | [<img src="demo/review-demo-spawn-view.png" alt="spawn view" width="200">](demo/review-demo-spawn-view.png) | ☑ | Spawn view: north sidewalk, the hero façade across the road |
-| `demo/review-demo-street-level.png` | [<img src="demo/review-demo-street-level.png" alt="street level" width="200">](demo/review-demo-street-level.png) | ☑ | Street level: portals, flights, banners, colonnade |
-| `demo/review-demo-crossing.png` | [<img src="demo/review-demo-crossing.png" alt="crossing" width="200">](demo/review-demo-crossing.png) | ☑ | The designated mid-block crossing (s=130 m): zebra markings on the road, kerbs flushed to ramps. Camera on the north sidewalk looking south-west. Note: the road renders near-black in shadow — the fixed-daytime-lighting item, not the markings |
-| `demo/review-demo-north-buildings.png` | [<img src="demo/review-demo-north-buildings.png" alt="north buildings" width="200">](demo/review-demo-north-buildings.png) | ☑ | North-side background buildings (TCL, Tussaud's) across the Blvd |
+| `demo/review-demo-spawn-view.png` | [<img src="demo/review-demo-spawn-view.png" alt="spawn view" width="200">](demo/review-demo-spawn-view.png) | needs review | Spawn view from the north sidewalk: the hero façade across the road under fixed daytime light |
+| `demo/review-demo-street-level.png` | [<img src="demo/review-demo-street-level.png" alt="street level" width="200">](demo/review-demo-street-level.png) | needs review | Street level from the north sidewalk: the façade colonnade and columns at eye height, now sunlit with cast shadows (was backlit and near-black) |
+| `demo/review-demo-crossing.png` | [<img src="demo/review-demo-crossing.png" alt="crossing" width="200">](demo/review-demo-crossing.png) | needs review | The designated mid-block crossing at s=130 m (UE X -14956): three continental zebra bars on the dark asphalt, kerbs flushed to ramps, camera on the north sidewalk looking along the crosswalk |
+| `demo/review-demo-north-buildings.png` | [<img src="demo/review-demo-north-buildings.png" alt="north buildings" width="200">](demo/review-demo-north-buildings.png) | needs review | North-side background buildings (TCL, Tussaud's) across the Blvd, lit against the sky |
 
 ## What to look for