PeriodicBlockStepper.step(n): wait on the stream once per call

Same change as the reviewed FixedPatchStepper end-sync (cherry-picked
ad1d82c+512dcc9 above): the n steps are enqueued in order and the host
waits once; step(0) launches nothing; step_count advances after the wait
and keeps its entry value on failure. Launches, buffer roles and bits are
those of n step(1) calls. Mocked-driver host test covers the single
wait, launch/role equality with n step(1), step(0) and failure.

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

src/CelerisLab/lbm/block_stepper.py

@@ -204,11 +204,18 @@ class PeriodicBlockStepper:
                 raise
 
     def step(self, n=1):
-        """Advance n steps: halo refresh, then core pull/collide, per step."""
+        """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.
+        """
         if isinstance(n, bool) or not isinstance(n, Integral) or n < 0:
             raise ValueError("n must be a nonnegative integer")
         with self._lock, self.ctx.activate():
             self._require("ready")
+            if n == 0:
+                return
             kw = dict(block=self._block, stream=self._stream)
             try:
                 self._status = "advancing"
@@ -224,9 +231,9 @@ class PeriodicBlockStepper:
                     self._step(self._current, self._spare, np.uint32(self.layout.halo),
                                self._tables.get("flags", np.uintp(0)), np.uintp(0),  # no kind
                                grid=self._grid, **kw)
-                    self._stream.synchronize()
-                    self._current, self._spare = self._spare, self._current
-                    self._step_count += 1
+                    self._current, self._spare = self._spare, self._current  # host handles only
+                self._stream.synchronize()
+                self._step_count += n
                 self._status = "ready"
             except BaseException:
                 self._status = "failed"