unit tests on _collisions need to stop at collisions9a now, not collisions9, or else it'll just be catching the end of the first loop, which does collisions for a monster, not the player.
fix actually reading the stick when flapping; when I moved the stick reading logic back in, I did put it somewhere where it didn't actually do anything.
also, fix the don't-go-below-0 checks in _momentum.  previously, if the player flapped to $80 or above, they'd escape gravity.  hmmm...