lbm: delete PeriodicBlockStepper; a single blocked level is FixedPatchStepper(refinements=[]) (S2)

The second owner of one blocked level goes. Its capability is the
zero-refinement FixedPatchStepper added in f764d68/985db14 (periodic SRT
level, stationary passive circles, shifted storage, NaN spare, single wait),
which matched it array for array on GPU at 1318fc2 (3 splits x +-circle x
shifted/unshifted) and passes the host lifecycle and fault contracts.

API change for callers:
  PeriodicBlockStepper(cfg, BlockLayout2D.build(ny, nx, by, bx), context=c, objects=o)
  -> FixedPatchStepper(cfg, context=c, refinements=[], level_blocks=[(by, bx)],
                       level_objects=[tuple(o)])
  download() returns a 1-tuple; static-circle tables are
  _level_boundaries[0].tables (was .boundary); no free-memory preflight.

- lbm/block_stepper.py deleted; MAX_GRID_DIM lives in refinement_stepper.
- tests/validation/multigrid/test_block_stepper.py: same frozen GPU gates
  (split identity, FP64 oracle, circle across seams) and pre-CUDA rejection
  cases on the single level; the old-vs-new bitwise test is retired with the
  old class (its result is the 1318fc2 record).
- tests/unit/test_block_step_sync.py folded into test_single_level.py
  (step(n) vs n x step(1) roles and one wait, step(0), mid-batch launch
  failure), which also gains bad-state rejection before upload, download
  failure and activation-failure retry on close.
- test_shifted_storage and test_raw_exchange_gpu use the single level; the
  raw-exchange reference feeds the AMR Jet column and raw ledger through the
  root level boundary instead of patching a private kernel field.
- Pinned historical runners (taylor_green/run_case.py, run_jet_circle_cv.py)
  are unchanged: they only run on trees that still ship PeriodicBlockStepper.
  No alias.

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

tests/unit/test_refinement_three_level_multipatch.py

@@ -22,11 +22,11 @@ def host_module():
     tree.body=[node for node in tree.body if not (
         isinstance(node,ast.Import) and any(a.name=='pycuda.driver' for a in node.names)
         or isinstance(node,ast.ImportFrom) and node.module in (
-            'cuda','block_stepper','curved_links'))]
+            'cuda','curved_links'))]
     module=types.ModuleType('CelerisLab.lbm._test_host_refinement')
     module.__package__='CelerisLab.lbm'
     from CelerisLab.body.jets import surface_link_velocity
-    module.__dict__.update(cuda=Mock(),compiler=Mock(),MAX_GRID_DIM=65535,
+    module.__dict__.update(cuda=Mock(),compiler=Mock(),
                            surface_link_velocity=surface_link_velocity)
     exec(compile(tree,str(path),'exec'),module.__dict__)
     return module