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.

TODO.md

@@ -87,7 +87,7 @@ The MPFB character asset is planned as the NPC template (see `ROADMAP.md`). Keep
 - [ ] Model surrounding block frontage.
 - [x] Model simplified background buildings directly across Hollywood Boulevard. Done 2026-11-07, the two in-scope north-side buildings — TCL Chinese Theatre (SM_TCL_Chinese_Theatre) and Madame Tussaud's (SM_Madame_Tussauds) — taken from the verified OSM blockout volumes and exported with `ue_metric_v1`, with a Y-mirror so they land north of Hollywood Blvd across the street from the hero (TCL spans Unreal Y 1802..9431, Tussaud's 1888..7586; road centreline ≈ 0, hero façade at Y −1450). Staged in the DemoMap as `North_TCL_Chinese` / `North_Madame_Tussauds`; demo shot `review-03-demo-north-buildings` captured. Simple flat facade materials (surfaces are the `0.1.4` patch).
 - [A] Build sidewalks, roads, and collision meshes. Terrain, road, sidewalks and kerbs exist (`Art/blender/terrain/`, `street/`) and are assembled in `site/site-2026-11-06.blend`; collision and details (crossing markings, kerb radii, aprons) are not built. Ground must follow the DEM: 3.94 m fall along the block, ~2 % crown between the gutters at ±10–11 m from the centreline, north sidewalk higher than south. Kerb height is an unresolved assumption — the DEM does not resolve it (`Geo/verification/2026-11-06-cross-sections.md`).
-- [A] Materials and look development for all exported assets.
+- [x] Materials and look development for all exported assets. Done 2026-11-07: every exported surface now carries a look-developed material built procedurally in `Tools/ue_build_demo_map.py` (`ensure_material`): base colour with a world-space noise blend between two shades for large-scale variation, plus an explicit roughness and specular. Set is `M_Asphalt` 0.92, `M_Concrete` 0.88, `M_KerbConcrete` 0.85, `M_Ground` 0.98, `M_RoadPaint` 0.62, `M_FacadeStone` 0.72 (hero façade) and `M_Stucco` 0.86 (TCL, Tussaud's), assigned by mesh and by actor label. Materials are rebuilt on every run so a stale asset cannot mask a look change. The richer texture-based pass (baked/procedural brick, brick_facade and seamless_concrete generators) stays in the `0.1.4` façade patch.
 
 ### Gameplay - active - 01a1b26b-3c94-707f-9665-2f6b4a79b564 (branch `gameplay-attribution-0.1.0`)