fix(vehicle): one chassis body, so the car sits on the road

The vehicle floated 3.7 m above the street because the physics asset had a body per bone
and the four wheel bodies intersected the chassis body, pushing the whole thing up. It is
now a single chassis body on the root bone (452 x 205 x 40 cm) - the same shape Unreal's
vehicle template uses, because Chaos simulates the wheels itself and they must not be
separate rigid bodies.

Settle check, same spawn point:
  before: settled at Z 494 cm
  after:  settled at Z 119 cm

Added -drivetest, which holds full throttle for 3 s and reports speed and distance. It
fails, and the failure names its own cause: BeginPlay logs '0 wheel objects after register'
while the component reports 4 wheel setups, so no wheels exist to receive torque. Recorded
in TODO.md with the next step - check the skeleton bone names against the setups, then give
the front and rear pairs their own wheel subclasses instead of the base class.

Game/GigLine/Source/GigLine/GigLineVehicle.h

@@ -36,6 +36,10 @@ private:
 	/** Configure the Chaos wheels from the modelled dimensions. */
 	void ConfigureWheels();
 
+	float DriveElapsed = 0.f;
+	bool bDriveReported = false;
+	FVector DriveStart = FVector::ZeroVector;
+
 	float SettleElapsed = 0.f;
 	bool bSettleReported = false;
 };