simulation: refresh the stepper latch only when a moving body is added or removed

Keeps the stationary add/remove path unchanged (test_surface_actuation builds
a Simulation without a stepper).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CelerisLab/simulation.py

@@ -172,6 +172,7 @@ class Simulation:
             self.bodies.stage_add(obj)
             return -1
         body_id = self.bodies.add(obj)
+        if translation is not None:
             self._refresh_stepper_block()
         return body_id
 
@@ -273,7 +274,9 @@ class Simulation:
         if self._initialized:
             self.bodies.stage_remove(id)
         else:
+            moving = self.bodies.get(id).motion is not None
             self.bodies.remove(id)
+            if moving:
                 self._refresh_stepper_block()
 
     # -- Legacy convenience (keep for backward compat) -----------------------