PeriodicBlockStepper.step: mid-batch launch failure test; async-error docstring note

Review follow-up (non-blocking items): a launch failure on the second of
three steps fails closed without the batch wait, keeps step_count at its
entry value and blocks download; the docstring now states, as for
FixedPatchStepper, that async faults surface at the single wait.

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

tests/unit/test_block_step_sync.py

@@ -78,6 +78,25 @@ class BlockStepSync(unittest.TestCase):
             run.step(2)
         self.assertEqual((run.step_count, run._status), (1, "failed"))
 
+    def test_launch_failure_mid_batch_fails_closed_without_waiting(self):
+        run, log = backend()
+        calls = []
+        step = run._step
+
+        def failing(*args, **kwargs):
+            calls.append(1)
+            if len(calls) == 2:
+                raise RuntimeError("launch failure")
+            return step(*args, **kwargs)
+
+        run._step = failing
+        with self.assertRaises(RuntimeError):
+            run.step(3)
+        self.assertNotIn("sync", [name for name, _ in log])
+        self.assertEqual((run.step_count, run._status), (0, "failed"))
+        with self.assertRaises(RuntimeError):
+            run.download()
+
 
 if __name__ == "__main__":
     unittest.main()