add FIXME about inclined orbits

src/simulation.cpp

@@ -187,6 +187,8 @@ static Vec3 calc_orbital_velocity(CelestialBody* body, CelestialBody* parent) {
     assert(v_squared >= 0);
     double speed = (double) sqrt(v_squared);
 
+    // FIXME: this whole section is just wrong and will break with inclined orbits
+    //  it's also a failure of our testing that we're not catching it
     Vec3 z_axis = {0.0, 0.0, 1.0};
     Vec3 vel_dir = vec3_cross(z_axis, r);