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>

src/CelerisLab/lbm/block_stepper.py

@@ -207,8 +207,11 @@ class PeriodicBlockStepper:
         """Advance n steps: halo refresh, then core pull/collide, per step.
 
         All n steps are enqueued in order on the one stream and the host waits
-        once before returning (same launches and bits as n step(1) calls). On a
-        failure the instance is failed and step_count keeps its entry value.
+        once before returning (same launches and bits as n step(1) calls).
+        Asynchronous device errors surface at that single wait, so they are not
+        attributed to the step that caused them; call step(1) repeatedly to
+        localize one. On any failure the instance is failed and step_count keeps
+        its entry value.
         """
         if isinstance(n, bool) or not isinstance(n, Integral) or n < 0:
             raise ValueError("n must be a nonnegative integer")