Just played 0.0.3 and I'm glad the glitches are largely fixed! It definitely feels much smoother to play already. :D
Yeah, I found precisely one core in my re-playthrough. Honestly, first time around, I just assumed you didn't actually put any cores in the demo and left them in there as a hint to what the rest of the game would have.
Part of the difficulty is that a lot of the world looks the same and I worried that I could possibly have been backtracking without realising it. Fortunately, that never happened.
I took delight in charging through the bridge once I got the boost upgrade. :D Then I took damage from two enemies at the same time mid-boost and my health dropped by two fifths. QnQ I believe it was when two of those small jumping enemies fired a shock ball at the same time.