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>

tests/unit/test_jets.py

@@ -300,3 +300,13 @@ def test_fine_boundary_keeps_edge_frame_hits_and_checks_jets():
         refinement.prepare_fine_boundary(cfg, patch, [legacy], jets=[jet])
     disjoint = Jet(Sector(C, math.pi, .5, R + .01), Radial(C, inward=False))
     refinement.prepare_fine_boundary(cfg, patch, [legacy], jets=[disjoint])
+
+
+def test_action_upload_keeps_cfg_none_fallback(device):
+    # Default ObjectManager(cfg=None) keeps the 2D slot fallback, empty or not.
+    from CelerisLab.body.manager import ObjectManager
+    stream = SimpleNamespace(synchronize=lambda: None)
+    bodies = ObjectManager(12, 10, 1, 9)
+    bodies._upload_action_async(stream)
+    bodies.add(circle(radius=2.))
+    bodies._upload_action_async(stream)