fix(vehicle): the car is red in the engine, not black

Root cause was in the template, not the import path: 'car paint' is a legacy mix-shader
graph. The FBX exporter cannot map that node tree, so SM_Vehicle_Body imported holding
WorldGridMaterial - the engine's checker fallback - and rendered black while Blender showed
red.

Two changes:
  - Tools/vehicle_from_template.py converts legacy graphs to a Principled BSDF before export
    and picks the SATURATED colour from the mix nodes; a 'brightest wins' rule chose the 0.8
    grey diffuse beside the 0.49 red and produced a grey car.
  - Tools/ue_vehicle_materials.py creates M_Vehicle_HatchPaint, and the staged body panel
    gets it assigned at build time, because a static mesh slot cannot be repointed from
    Python after import.

Judge on the new capture: verdict=yes confidence=0.9 - 'a red hatch car with a red body and
dark wheels parked on a road'. The comparison table in Screenshots/README.md now shows both
columns agreeing on colour.

Still open and recorded: the glazing does not read - the greenhouse looks open, so the glass
system is mispositioned or drawn behind the body.

Tools/ue_build_demo_map.py

@@ -719,6 +719,14 @@ def build():
             if comp:
                 comp.set_static_mesh(mesh)
                 comp.set_mobility(unreal.ComponentMobility.MOVABLE)
+                # The body imports with no usable material (its slot resolves to
+                # WorldGridMaterial), so the paint is assigned on the staged panel.
+                if mesh_name == "SM_Vehicle_Body":
+                    paint = unreal.EditorAssetLibrary.load_asset(
+                        "/Game/Hollywood/Vehicle/M_Vehicle_HatchPaint")
+                    if paint:
+                        comp.set_material(0, paint)
+                        log("Demo_VehiclePanel: body paint assigned")
             staged_panels += 1
     log(f"staged: {staged_panels} Demo_VehiclePanel actors (plain static meshes)")