lighting: fixed daytime rig, real surface materials, dynamic lights

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Completes "Fixed daytime lighting and basic ambience" for 0.1.0. All 14
screenshots approved by the owner.

Lighting rig (Tools/ue_build_demo_map.py):
- Sun pitch -50 / yaw 120, intensity 4, warm (255/245/230), atmosphere sun light
- Sky light 2.6 with real-time capture, sky atmosphere, height fog (density 0.008)
- Unbound post-process volume with fixed exposure (min == max brightness) so a
  shot cannot wash out or crush to black depending on how much sky it frames
- Dynamic-only: r.AllowStaticLighting=False plus MOVABLE lights and mesh
  components, so the editor's "LIGHTING NEEDS TO BE REBUILT" warning no longer
  fires and nothing needs baking

Bugs found and fixed on the way:
- Blender diffuse colours do not survive the FBX -> Unreal import: every site
  material arrived white and the terrain arrived with WorldGridMaterial, which
  is what actually made the scene look blown out. The demo builder now creates
  and assigns M_Asphalt, M_Concrete, M_Ground and M_RoadPaint.
- Only one of the three crossings reached the game: each zebra band was its own
  object, so Blender renamed the duplicates .001/.002 and exact-name site
  assembly skipped them. All bands are now one Crossing_Markings mesh.
- Demo cameras: street-level sat inside the facade body (rendering its unlit
  back face) and the crossing camera aimed at the s=36.6 crossing instead of
  the designated s=130.1 (UE X -14956).
- The terrain had no material and imported as a bright default grid.
- Cleared a committed Android File Server SecurityToken (plugin disabled and
  shipping-excluded; the value must stay empty).

Docs:
- Screenshots/README.md: one consolidated table for all 14 shots with map,
  camera view and description; all approved.
- MEMORY.md: records the material-import, exposure, light_color struct,
  band-naming, camera-bounds and lighting-build traps.
- TODO.md: lighting item marked done.

Squash of branch gameplay-lighting-0.1.0 (PR #3).

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