Iteration: Jump bars and input arrows

Doing some iteration on the core gameplay loop.

I've added "jump bars" alongside adjacent cars. When the jump arrow reaches a jump bar, you can jump on that car. Should be simple and clear, I hope.

I've also added immediate input feedback. This consists of a green arrow when you successfully issue a jump command, and a red X when you issue a jump that can't currently be performed.

Hopefully this makes it play better for new players.