materials: look-developed surface materials for every exported asset

Replaces the flat Constant3Vector fills with a procedural material set built in
ensure_material(): base colour blended between two shades by world-space noise
for large-scale variation, plus explicit roughness and specular so each surface
type reads.

- M_Asphalt 0.92, M_Concrete 0.88, M_KerbConcrete 0.85, M_Ground 0.98,
  M_RoadPaint 0.62, M_FacadeStone 0.72, M_Stucco 0.86
- Site meshes take their material by mesh name; the hero facade and the two
  north-side buildings take theirs by actor label, so they are covered even
  though other scripts stage them.
- Materials are deleted and rebuilt on every run, so a look change cannot be
  masked by a stale asset.

Screenshots re-rendered: the facade reads as warm stone with surface variation,
asphalt as dark asphalt with grain, concrete and kerbs lighter, and the north
buildings in stucco instead of flat grey.

Docs: Screenshots/README documents the material set; TODO marks the item done
and leaves the richer texture-based pass to the 0.1.4 facade patch.

Screenshots/README.md

@@ -50,10 +50,14 @@ All ☑ approved 2026-11-07.
   Z 2 m; both are exactly 1 m — the scale reference.
 - **Demo map** (`/Game/Hollywood/DemoMap`): fixed daytime — sun at pitch −50 /
   yaw 120 (intensity 4, warm), sky light 2.6, sky atmosphere, height fog, fixed
-  exposure. 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.
+  exposure.
+- **Materials**: every exported surface carries a look-developed material built
+  in `Tools/ue_build_demo_map.py` (`ensure_material`) because Blender's flat
+  diffuse colours do not survive the FBX → Unreal import. Each has an explicit
+  roughness and a subtle world-space noise blend between two shades:
+  `M_Asphalt` (0.92), `M_Concrete` (0.88), `M_KerbConcrete` (0.85),
+  `M_Ground` (0.98), `M_RoadPaint` (0.62), `M_FacadeStone` (0.72) and
+  `M_Stucco` (0.86) for the north-side buildings.
 - **Lighting is dynamic-only** (`r.AllowStaticLighting=False`, all lights
   Movable), so no lighting build is ever required.
 - **Attribution overlay**: every shot carries the required ODbL credit in the