docs/tests: state the S3a commit point; cover a failed hold-record upload

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

tests/unit/test_moving_bodies.py

@@ -254,7 +254,8 @@ class Tables:
     def upload_records(self, circles, stream):
         records = [m.record() for m, _ in circles]
         self.log.append(("upload", records))
-        if self.fail == "upload":
+        uploads = sum(1 for w in self.log if isinstance(w, tuple) and w[0] == "upload")
+        if self.fail == "upload" or (self.fail == "hold_upload" and uploads == 2):
             raise RuntimeError("upload failed")
         return any(r[0] != r[1] for r in records)
 
@@ -294,15 +295,18 @@ 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"))
-def test_failed_advance_commits_nothing_and_fails_simulation(monkeypatch, failure):
-    kwargs = dict(fail="upload") if failure == "upload" else dict(no_donor=1.)
+@pytest.mark.parametrize("failure", ("upload", "no_donor", "hold_upload"))
+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))
     with pytest.raises(RuntimeError):
         sim.run(1, stream=STREAM)
-    assert motion.applied == (0., 0.) and motion.failed and sim._failed is not None
+    # 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.))
+    assert motion.failed and sim._failed is not None
     with pytest.raises(RuntimeError, match="Simulation failed"):
         sim.run(1, stream=STREAM)