Solid game! I'd recommend also showing (Esc to go back) in the instructions and at the start of a run. For the full game maybe have gems carry over between runs/do more than just heart packs (upgrades to gun/speed/airtime etc maybe?) and it'd be really helpful to be able to speed up or slow down and adjust my position on the screen horizontally even if just a little to avoid frustrating deaths i can see coming but would unintuitively need to use the sword to stall my momentum or something.
Great job!