fix: render prograde orbits counter-clockwise by removing sign flip

Added test case validating prograde motion in simulation frame.
Fixed sim_to_render coordinate transform to remove negative sign
on Z component, which was causing clockwise visual appearance
for prograde orbits when viewed from above (+Y axis).

src/renderer.cpp

@@ -239,7 +239,7 @@ void update_camera(RenderState* render_state, SimulationState* sim) {
 Vector3 sim_to_render(Vec3 pos, double scale) {
     return (Vector3){
         (float)(pos.x * scale),
-        (float)(-pos.z * scale),
+        (float)(pos.z * scale),
         (float)(pos.y * scale)
     };
 }

tests/test_orbital_period.cpp

@@ -70,3 +70,33 @@ TEST_CASE("Orbital period - Mars (RK4)", "[period][rk4]") {
     destroy_orbit_tracker(tracker);
     destroy_simulation(sim);
 }
+
+TEST_CASE("Orbit direction - prograde for zero inclination", "[direction]") {
+    const double TIME_STEP = 60.0;
+    const double TEST_DURATION_DAYS = 1.0;
+    const double SECONDS_PER_DAY = 86400.0;
+    const int STEPS = (int)(TEST_DURATION_DAYS * SECONDS_PER_DAY / TIME_STEP);
+
+    SimulationState* sim = create_simulation(2, 0, 0, TIME_STEP);
+    REQUIRE(load_system_config(sim, "tests/test_energy.toml"));
+
+    CelestialBody* sun = &sim->bodies[0];
+    CelestialBody* earth = &sim->bodies[1];
+
+    Vec3 initial_rel_pos = vec3_sub(earth->global_position, sun->global_position);
+    double theta_start = atan2(initial_rel_pos.y, initial_rel_pos.x);
+
+    for (int i = 0; i < STEPS; i++) {
+        update_simulation(sim);
+    }
+
+    Vec3 final_rel_pos = vec3_sub(earth->global_position, sun->global_position);
+    double theta_final = atan2(final_rel_pos.y, final_rel_pos.x);
+
+    INFO("Initial angle: " << theta_start << " rad");
+    INFO("Final angle: " << theta_final << " rad");
+
+    REQUIRE(theta_final > theta_start);
+
+    destroy_simulation(sim);
+}