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.

TODO.md

@@ -24,6 +24,8 @@ Actionable task list for this project. This file is the single, central place fo
 - [x] Fix the vehicle not rendering in the scene. **Chased to a strong clue, not yet solved.** The vehicle is staged and runs (`spawned at (-12950,-900,120)`, `9 visual panels attached`, `panel 0 at (-12950,-900,120) visible=1 extent=238`, `settled at Z 119`), and the review capture shows an empty road with only a soft shadow. `Tools/visual_loop.py --verify` passes the frame; `Tools/judge_screenshot.py` returns `verdict=no confidence=1.0`. A downward trace from the vehicle finds **no ground at all** (`ground at Z -9999`), so the car is over a surface without collision or inside geometry - the shadow and the panels are not in the same place. **Fixed 2026-11-09** — the car renders, built from the owner's `hot hatch.blend`. **Fixed:** the vehicle was spawned 2.5 m under the road - a downward trace measured the surface at Z 366 cm against a spawn of Z 120, so the body was buried and only its shadow showed. The spawn is now Z 470 and it settles there.
   - **Still open:** above ground, the capture still shows road plus a shadow and no car. Panels report `visible=1`, `mesh=SM_Vehicle_Body`, extent 238 cm, at the expected position; `MarkRenderStateDirty()` and `UpdateBounds()` run after registration. Next suspects: the pawn's own `SetActorHiddenInGame` path (a vehicle pawn is not a plain actor), or the panels being attached to a root that the physics simulation moves independently of the actor transform.
 - [x] Fix the character and car floating. Character: the player start at Z 600 left the soles 39 cm in the air (the capsule settles at 561 on this pavement); the start is now the measured 561. Car: the template's lowest geometry is exhaust, so grounding on the whole model put the tyres 4 cm above the origin - it is now grounded on its wheels, staged at Z 342 after the traced road height (366) disagreed with the visible road by ~25 cm.
+- [x] Fix the vehicle's material colour. The template's `car paint` is a legacy mix-shader graph that the FBX exporter cannot map, so `SM_Vehicle_Body` imported holding `WorldGridMaterial` and rendered black while Blender showed red. Legacy graphs are now converted to Principled before export (saturated colour wins over the grey diffuse next to it), and the staged body panel is assigned `M_Vehicle_HatchPaint`. Judge: `verdict=yes confidence=0.9`.
+- [ ] Fix the missing vehicle glazing. The greenhouse reads as open from the review angle: the glass system is mispositioned or drawn behind the body.
 - [ ] Implement enter/exit interaction.
 - [ ] Build a closed driving route with boundaries.
 - [ ] Tune basic handling to a drivable state.