Fix parent body transition bug in simulation

When bodies change gravitational parents, properly update their
local coordinates to maintain physics accuracy. This fixes the
bug where local positions were never updated when changing parents.

Also remove unused variable warning.

src/simulation.cpp

@@ -85,9 +85,22 @@ void update_simulation(SimulationState* sim) {
 
         int new_parent = find_dominant_body(sim, i);
         if (new_parent != body->parent_index && new_parent != -1) {
-            // FIXME: there's now a bug here when changing parent bodes
-            //  I think it's that the local position is never updated
+            // Convert current local coordinates to global coordinates using old parent
+            if (body->parent_index >= 0 && body->parent_index < sim->body_count) {
+                CelestialBody* old_parent = &sim->bodies[body->parent_index];
+                body->position = vec3_add(body->local_position, old_parent->position);
+                body->velocity = vec3_add(body->local_velocity, old_parent->velocity);
+            }
+            
+            // Update parent index
             body->parent_index = new_parent;
+            
+            // Convert global coordinates to local coordinates using new parent
+            if (body->parent_index >= 0 && body->parent_index < sim->body_count) {
+                CelestialBody* new_parent_body = &sim->bodies[body->parent_index];
+                body->local_position = vec3_sub(body->position, new_parent_body->position);
+                body->local_velocity = vec3_sub(body->velocity, new_parent_body->velocity);
+            }
         }
 
         if (body->parent_index >= 0 && body->parent_index < sim->body_count) {
@@ -109,7 +122,7 @@ static void compute_orbital_velocity_from_vis_viva(CelestialBody* body,
                                                      CelestialBody* parent) {
     Vec3 r = vec3_sub(body->position, parent->position);
     double distance = vec3_magnitude(r);
-    double e = body->eccentricity;
+    // double e = body->eccentricity; // FIXME: unused variable
     double a = body->semi_major_axis;
     double v_squared = G * parent->mass * (2.0 / distance - 1.0 / a);
     // FIXME: need error handling here