Nice! But how about trying for the “extra points”? To quote:
* Bonus points if your submission fits in one tweet 😉
(Of course, I’m just joking! What sort of demo could you write that only took 140 characters. Maybe a “Hello WORLD”!

A proper suggestion: You reset the spawn cooldown r.f=s.f=10
If you wanted to save a byte, how about putting it down to r.f=s.f=9
Not sure how that’ll affect your logic, though.
(Of course, you could save another byte by allowing spawing when r.f > 0 rather than r.f == 0 . Again, this’ll affect the cooldown period…)

Nicholas