simulation: derive the stepper latch from moving bodies on add/remove

One helper sets stepper.blocked from _moving_bodies() after add_body,
pre-init remove_body, initialize and stepper recreation, so the latch never
disagrees with motion ownership. Tests: latch follows add/remove; the
after-close assertion checks the actual refusal.

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

src/CelerisLab/simulation.py

@@ -148,7 +148,6 @@ class Simulation:
                 from .body.motion import MovingCircle
                 obj.motion = MovingCircle((cx, cy), radius, translation,
                                           on_failure=self._fail_moving)
-                self.stepper.blocked = MOVING_BLOCKED
         elif type_lower == "sensor":
             from .body.geometry.circle import CircleGeometry
             cx, cy = float(center[0]), float(center[1])
@@ -172,7 +171,9 @@ class Simulation:
         if self._initialized:
             self.bodies.stage_add(obj)
             return -1
-        return self.bodies.add(obj)
+        body_id = self.bodies.add(obj)
+        self._refresh_stepper_block()
+        return body_id
 
     def add_jet(self, jet) -> int:
         """Register a body-independent Jet before ``initialize()``; returns its index.
@@ -212,6 +213,10 @@ class Simulation:
         # Device geometry is unknown after a failed moving step: same lifecycle as Jets.
         self._failed = f"moving circle step failed: {exc!r}"
 
+    def _refresh_stepper_block(self) -> None:
+        # Derived latch: direct stepper calls must not step after static-link exclusion.
+        self.stepper.blocked = MOVING_BLOCKED if self._moving_bodies() else None
+
     def _moving_bodies(self):
         return [obj for obj in self.bodies.objects if obj.motion is not None]
 
@@ -269,6 +274,7 @@ class Simulation:
             self.bodies.stage_remove(id)
         else:
             self.bodies.remove(id)
+            self._refresh_stepper_block()
 
     # -- Legacy convenience (keep for backward compat) -----------------------
     def add_cylinder(self, center: Tuple[float, ...],
@@ -516,7 +522,7 @@ class Simulation:
             self.field, self._module, self.lbm_cfg,
         )
         self.stepper._step_count = _prev_step_count
-        self.stepper.blocked = MOVING_BLOCKED if self._moving_bodies() else None
+        self._refresh_stepper_block()
 
         self._assert_object_count_contract(expected_count=expected_n_objects)
 
@@ -543,7 +549,7 @@ class Simulation:
         # change. Compact-list construction publishes no outputs on failure.
         next_flags = self.bodies.build_flags(self.field.build_channel_flags())
         self._validate_moving_bodies()
-        self.stepper.blocked = MOVING_BLOCKED if self._moving_bodies() else None
+        self._refresh_stepper_block()
         if self.bodies.jets and self.bodies.count == 0:
             raise ValueError("Jets need bodies with cut links")
         self.bodies.build_compact_lists_for(self.bodies.objects, next_flags)

tests/unit/test_moving_bodies.py

@@ -232,6 +232,16 @@ def test_add_body_and_set_body_route_targets(monkeypatch):
                          **kwargs)
 
 
+def test_stepper_latch_follows_moving_bodies(monkeypatch):
+    sim = routed_simulation(monkeypatch)
+    still = sim.add_body("circle", (100., 40.), 5.)
+    assert sim.stepper.blocked is None
+    moving = sim.add_body("circle", (50., 40.), 10., translation=Translation((0., 0., -1., 1.), .1))
+    assert sim.stepper.blocked is not None
+    sim.remove_body(moving)  # removing the last translating circle clears the latch
+    assert sim.stepper.blocked is None and sim.bodies.get(still).motion is None
+
+
 def validate(monkeypatch, objects, flags=None, **cfg):
     sim = routed_simulation(monkeypatch)
     for obj in objects:
@@ -306,5 +316,5 @@ def test_failed_moving_step_marks_simulation_failed(monkeypatch):
     with pytest.raises(RuntimeError, match="unknown"):  # no retry on the failed circle
         motion.advance(lambda *_: None, lambda: None, lambda: None)
     sim.close()  # close still works
-    with pytest.raises(Exception):  # no re-entry after close
+    with pytest.raises(RuntimeError, match="Simulation failed"):  # no re-entry after close
         sim.run(1)