Merge periodic stepper alias cleanup into final stage candidate

docs/compatibility.md

@@ -3,6 +3,13 @@
 Changes that can make a previously accepted setup or call behave differently.
 Each entry names the commit, the affected entry points and the observable result.
 
+## Redundant periodic stepper alias removed
+
+`PeriodicPatchStepper` is no longer exported from `lbm.refinement_stepper`.
+Use `FixedPatchStepper` with the same arguments, including explicit
+`level_blocks`. The old name was an alias for that exact class; periodic
+boundaries remain the default. Existing imports of the old name must be updated.
+
 ## 87c3a31: cut-link ownership is validated for every body
 
 `validate_cut_link_ownership` (`body/preprocess/flag_overlay.py`) now runs for