Solve for a target drive-away price

Works back from a drive-away price. Send a normal quote request plus a target; the engine finds the largest value for one dutiable line (by default the first, i.e. the vehicle price), on a whole-increment grid, whose drive-away does not exceed the target, and returns the full quote at that value. Every statutory charge is recomputed at the solved value. Duty is stepped (per $100 or $200), so an exact match is not always possible: solution.shortfall reports the gap, which is never negative (the quote never exceeds the target).