fix(vehicle): settle the chassis on the road (float 33 m -> 6 cm)

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Two root causes, both in how the chassis box was authored:

1. The box centre was -Extent.Z*0.25, below the root bone, so the box sat under the
   ground and the suspension pushed the whole vehicle up. The skeletal mesh imports
   ROLLED (height along Y, width along Z) while the armature is unrolled, so the box is
   now built with swapped axes (length from mesh X, width from mesh Z, height from mesh
   Y) and centred at the rolled mesh's mid-height (Bounds.Origin.Y). The vehicle now
   settles at Z 6 cm instead of floating at 33 m.

2. A left-over diagnostic stopped the movement component at 2.5 s whenever the pawn
   ticked, which cut the drive test (throttle runs 1-4 s). It is now gated on the vehicle
   actually having zero wheels.

Remaining: the drive test still reports ~0 forward speed, so the throttle does not yet
move the car - the wheels rest ~2 cm above the ground because the suspension rest
position is measured against the root bone while the box sits at the centre of mass.

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