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>

docs/lifecycle.md

@@ -1,4 +1,4 @@
-# Device state lifecycle (uniform Simulation, PeriodicBlockStepper, FixedPatchStepper)
+# Device state lifecycle (uniform Simulation, FixedPatchStepper)
 
 State is grouped by what a change invalidates. This file describes the code as
 it is; it adds no framework. Line references are to candidate 0f8fdbc3.
@@ -13,8 +13,8 @@ it is; it adds no framework. Line references are to candidate 0f8fdbc3.
 Storage routes are separate concerns:
 
 - Original uniform grid (`uniform.cu`, `Simulation`): whole-domain `NX`/`NY`.
-- Blocked levels (`blocked.cu`; `PeriodicBlockStepper` and every blocked
-  `FixedPatchStepper` level): `NX`/`NY` are one block window including halo.
+- Blocked levels (`blocked.cu`; every `FixedPatchStepper` level, including the
+  single level of `refinements=[]`): `NX`/`NY` are one block window including halo.
 - Dense refinement levels (`refinement.cu`, `level_blocks[l] is None`).
 
 A single-rectangle refinement (`FixedPatch2D`) is stored in its own fine