refinement_stepper: compatibility views are kept on purpose (comment only)

The views and the PeriodicPatchStepper alias stay for the existing tests
and docs (about 130 references); the comment no longer says REMOVE AFTER.
No code change.

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

src/CelerisLab/lbm/refinement_stepper.py

@@ -949,8 +949,8 @@ class FixedPatchStepper:
             if exc[0] is None:
                 raise
 
-    # Read-only compatibility views over the level lists. Internal code must not use them;
-    # REMOVE AFTER the existing tests/docs migrate to hierarchy/_layouts/_level_* lists.
+    # Read-only compatibility views over the level lists, kept on purpose for the existing
+    # tests and docs. Internal code must not use them; it reads hierarchy/_layouts/_level_*.
     patch = property(lambda self: self.hierarchy.pairs[0])
     child_patch = property(lambda self: self.hierarchy.pairs[1] if self.hierarchy.depth > 2 else None)
     base_layout = property(lambda self: self._layouts[0])