simulation: a failed moving step also blocks direct stepper calls

Tests cover failures at the first upload, the step-group sync, the no_donor
check, the hold-record upload and its asynchronous sync: before the commit
point the target stays pending, after it applied matches the moved device;
in every case run and direct stepper reuse are refused.

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

tests/unit/test_moving_bodies.py

@@ -295,18 +295,28 @@ def test_run_one_advances_and_commits_after_sync(monkeypatch):
     assert ("step", 3, False) in writes and motion.applied == (0., .25)
 
 
-@pytest.mark.parametrize("failure", ("upload", "no_donor", "hold_upload"))
+@pytest.mark.parametrize("failure", ("upload", "first_sync", "no_donor", "hold_upload",
+                                     "hold_sync"))
 def test_failed_advance_fails_simulation(monkeypatch, failure):
     kwargs = dict(no_donor=1.) if failure == "no_donor" else dict(fail=failure)
     sim, writes, motion = advancing_simulation(monkeypatch, **kwargs)
     motion._on_failure = sim._fail_moving
     motion.set_target((0., .25), (0., .05))
+    syncs = []
+
+    def synchronize():  # stands in for an asynchronous failure surfacing at sync
+        syncs.append(1)
+        if (failure, len(syncs)) in (("first_sync", 1), ("hold_sync", 2)):
+            raise RuntimeError("sync failed")
     with pytest.raises(RuntimeError):
-        sim.run(1, stream=STREAM)
-    # Before the step-group sync nothing is committed; a failed hold-record upload
-    # happens after the commit (applied already matches the moved device geometry).
-    assert motion.applied == ((0., .25) if failure == "hold_upload" else (0., 0.))
+        sim.run(1, stream=SimpleNamespace(synchronize=synchronize))
+    # Before the step-group sync and no_donor check nothing is committed; the hold
+    # record is staged after the commit (applied already matches the moved device).
+    committed = failure in ("hold_upload", "hold_sync")
+    assert motion.applied == ((0., .25) if committed else (0., 0.))
+    assert (motion.pending is None) == committed
     assert motion.failed and sim._failed is not None
+    assert sim.stepper.blocked == sim._failed  # direct stepper reuse refused too
     with pytest.raises(RuntimeError, match="Simulation failed"):
         sim.run(1, stream=STREAM)