This looks cool, I read a little of the code and it seems like good C code written by a sensible person in the current decade.

One suggestion: I noticed your nav.c depends on render.h - selfishly, as an AI researcher, I want more games/simulations where it's possible to run the core simulation without expecting to render to a screen or to read input from the real keyboard/mouse. So, if you can structure the engine such that it can run "headless" like this, that would be nice :)