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

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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.

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