refinement: single level matches PeriodicBlockStepper lifecycle; host contract tests

Review follow-ups for S1 (f764d68):
- a single level starts its spare buffer NaN, as PeriodicBlockStepper does,
  so stale halos cannot hide (multilevel initialization unchanged);
- projected explosion is rejected for a single level;
- new tests/unit/test_single_level.py: NaN spare after initialize, per-step
  launch order halo -> BFL (circles only) -> BlockStep with a null kind and
  one wait per step(n), circle solids excluded from the state mask;
- the GPU bitwise comparison also covers shifted storage.

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

tests/validation/multigrid/test_block_stepper.py

@@ -130,6 +130,7 @@ class CpuContracts(unittest.TestCase):
                         (config(collision="MRT"), {}),
                         (None, dict(observe_raw_exchange=True)),
                         (None, dict(level_jets=[(object(),)])),
+                        (None, dict(explosion="projected_linear")),
                         (None, dict(level_objects=[(circle((15.5, 4.0), 4.3),)]))):  # touches edge
             with self.subTest(kw=kw), self.assertRaises(ValueError):
                 single(cfg, **kw)
@@ -354,16 +355,17 @@ class PeriodicBlockGPU(unittest.TestCase):
         from CelerisLab.cuda.context import CudaContext
         g0 = initial_state()
         with CudaContext(0) as context:
-            for (by, bx), objects in [(split, objects) for split in SPLITS
-                                      for objects in ((), (circle(),))]:
-                with self.subTest(split=(by, bx), bodies=len(objects)):
+            for (by, bx), objects, shifted in [(split, objects, shifted) for split in SPLITS
+                                               for objects in ((), (circle(),))
+                                               for shifted in (False, True)]:
+                with self.subTest(split=(by, bx), bodies=len(objects), shifted=shifted):
                     layout = BlockLayout2D.build(NY, NX, by, bx)
-                    with PeriodicBlockStepper(config(), layout, context=context,
+                    with PeriodicBlockStepper(config(ddf_shifting=shifted), layout, context=context,
                                               objects=objects) as backend:
                         backend.initialize(g0)
                         backend.step(STEPS)
                         expected = backend.download()
-                    with FixedPatchStepper(config(), context=context, refinements=[],
+                    with FixedPatchStepper(config(ddf_shifting=shifted), context=context, refinements=[],
                                            level_blocks=[(by, bx)],
                                            level_objects=[objects]) as backend:
                         backend.initialize(g0)