Action upload: keep 2D slot fallback for ObjectManager(cfg=None)

19bd308 computed slot = 3*self.cfg.dim unconditionally, so a default
manager without cfg raised even when empty. Use the existing fallback
(cfg.dim if cfg else 2). Regression for empty and one-body managers.

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

src/CelerisLab/body/manager.py

@@ -627,7 +627,7 @@ class ObjectManager:
         Single gate for every action path (setters or direct ``action`` edits):
         surface-actuated and Jet-driven bodies must keep zero motion slots.
         """
-        slot = 3*self.cfg.dim
+        slot = 3*(self.cfg.dim if self.cfg else 2)
         for obj in self.objects:
             obj.validate_surface_actuation(self.cfg)
             base = obj.obj_id*slot