After much reading, internet googling and experimentation, I've been able to implement quaternions as the basis for ship orientation and movement.

Still lots of work to do though, as many items (such as the shot spawn routine, and enemy analysis of which direction to move for attack) are still Euler-angle based. For now, though, v0.09 is the first version which allows true 3D movement, uninhibited by gimbal lock.

My goal for version 0.10: to have it as fully functional as the last Euler-angle based version, with shots and enemy AI all working normally but using quaternions.