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

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).

MEMORY.md

@@ -174,3 +174,14 @@ Scope: facts and traps that cost time to establish. Planned work lives in [TODO.
 - **`EditorLevelLibrary.load_level`/`new_level` are deprecated no-ops in `-nullrhi` commandlets** (they return without switching worlds, so staged actors land in whatever world is current and get saved over the wrong map). Use the `LevelEditorSubsystem`: `unreal.get_editor_subsystem(unreal.LevelEditorSubsystem).load_level(path)` actually switches. Symptom was ReviewMap containing Demo actors + `ShotSequencer`.
 - **Map-save ordering bug in `Tools/ue_build_review_map.py`**: the script saved ReviewMap *after* the DemoMap section, but the switch back to ReviewMap fails silently because the map only exists in memory at that point — so the final `save_map` wrote the demo world over ReviewMap. Fix: save ReviewMap first, then stage the demo cameras, and never switch back. Both maps verified by probe (ReviewMap lists Review actors; DemoMap lists Demo actors).
 - Rebuilt ReviewMap after the fixes: character, both cubes (A grounded, B floating at 2 m), pawn, NPC prefab, hero facade all present; user re-approved the shots. README cube naming corrected (README had A/B swapped vs the script).
+
+## 2026-11-07 — Materials and lighting in the demo map
+
+- **Blender flat diffuse colours do not survive FBX -> Unreal.** Every site mesh (road, sidewalks, kerbs, crossing paint) imported with a material asset that had a default *white* base colour, and the terrain imported with `WorldGridMaterial` (UE's bright grey grid). That — not the sun — is what made the demo look blown out and washed white. Fix: create the colours in UE (`M_Asphalt`, `M_Concrete`, `M_Ground`, `M_RoadPaint` via `Constant3Vector`) and assign them per actor in `Tools/ue_build_demo_map.py`.
+- **Auto-exposure makes review shots non-comparable**: the same road rendered washed-out in one framing and near-black in another because each camera saw a different amount of sky. Fix: an unbound `PostProcessVolume` with `AEM_HISTOGRAM` and `auto_exposure_min_brightness == auto_exposure_max_brightness` (fixed exposure). Do not use `AEM_MANUAL` + `auto_exposure_bias` — a bias of 11 blew the whole frame out.
+- **`DirectionalLightComponent.light_color` takes a `Color` struct, not `LinearColor`** (`unreal.Color(r=255,g=245,b=230,a=255)`); passing `LinearColor` raises a nativize error.
+- **One band per object loses geometry downstream.** `crossing_band()` created `Crossing_Markings` each call, so Blender renamed the second and third to `.001`/`.002`; `site_assembly.py` appends by exact name, so only one crossing ever reached the game. Build all bands into a single mesh instead of one object per band.
+- **Street arc length -> UE X:** s=0 is the west end of the block (UE X −27966 cm) and s increases eastward, so the designated mid-block crossing at s=130.1 m is UE X ≈ −14956, not the −24291 used by an earlier camera (that is the s=36.6 crossing).
+- **Camera placement in the demo map needs the mesh bounds, not intuition.** The hero façade body spans UE Y −1457..−200, so a "street level" camera at Y −400 is *inside* the building and renders its unlit back face. Read `get_bounding_box()` on the staged meshes before choosing a camera.
+- **"LIGHTING NEEDS TO BE REBUILT" (Build Lighting Only / `BuildLighting`) appears whenever a map has Stationary/Static lights and no baked lightmap.** For a fully dynamic scene the clean fix is `r.AllowStaticLighting=False` in `Config/DefaultEngine.ini` plus `set_mobility(ComponentMobility.MOVABLE)` on the directional light, sky light and height fog (and MOVABLE on the staged mesh components). Nothing to build, warning gone. Note the cvar is read at startup, so each new commandlet/game run picks it up.
+- The Android File Server `SecurityToken` kept reappearing in `Config/DefaultEngine.ini`; the plugin is disabled and excluded from shipping, so the value is cleared and must stay empty — it is a credential and must not be committed.