materials: look-developed surface materials for every exported asset

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Completes "Materials and look development for all exported assets". Every
exported surface previously carried a flat Constant3Vector fill, which read as
plastic. ensure_material() in Tools/ue_build_demo_map.py now builds a
procedural material per surface:

- base colour blended between two shades by world-space noise
  (WorldPosition -> Multiply -> Noise -> Lerp -> BaseColor) for large-scale
  variation,
- an explicit roughness and specular so the surface type is readable.

  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.

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

Screenshots re-rendered and approved by the owner. The richer texture-based
pass (baked/procedural brick_facade, seamless_concrete, glass, material
instances) stays in the 0.1.4 facade patch.

Squash of branch materials-lookdev-0.1.0 (PR #5).

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