fix(vehicle): spawn on the road, not 2.5 m under it

Measured rather than guessed: a downward trace from the spawn point reports the road
surface at Z 366 cm, while the vehicle was spawned at Z 120. The car was buried; the only
thing visible in the capture was its shadow falling on the road above it. Spawn is now
Z 470, the camera aims at 430, and it settles at Z 470.

Not fixed, and recorded: with the car above ground the capture still shows road and a
shadow with no car. Panels register, report visible=1 with mesh=SM_Vehicle_Body and a
238 cm extent at the expected position, and MarkRenderStateDirty()/UpdateBounds() run
after registration. The shadow proves a collider is there; the visible geometry is not.

Screenshots/README.md now separates the two causes so the next person does not re-fix the
buried spawn.

Tools/ue_build_demo_map.py

@@ -652,7 +652,10 @@ def build():
     # configures the Chaos wheels from the modelled dimensions.
     vehicle_class = unreal.load_class(None, "/Script/GigLine.GigLineVehicle")
     if vehicle_class:
-        vehicle = spawn(vehicle_class, unreal.Vector(-12950.0, -900.0, 120.0),
+        # Spawn ON the road: a downward trace measured the surface at Z 366 cm there, so
+        # the previous Z 120 spawn buried the car 2.5 m inside the ground - which is why
+        # it was invisible in the capture while its shadow still fell on the surface.
+        vehicle = spawn(vehicle_class, unreal.Vector(-12950.0, -900.0, 470.0),
                         "Demo_Vehicle_Sedan")
         log(f"staged: Demo_Vehicle_Sedan ({'ok' if vehicle else 'FAILED'})")
     else: