blocks: one definition of a blocked level's config and kernel launches (C1a)

PeriodicBlockStepper and FixedPatchStepper each built the blocked level's
kernel config (block window NX/NY, neutralized wall/slot/inlet fields,
omega/tau check) and each spelled out the raw BlockHaloCopy and
BlockCurvedBoundary argument lists. These now live once in lbm/blocks.py
(level_config, launch_halo_copy, launch_curved_boundary), so a kernel ABI
change has one host site. Both owners keep their own lifecycle; no new
class, no behaviour or default change. PeriodicBlockStepper now checks its
level config before any device work (same errors). Dense refinement levels
keep their own config check.

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

tests/validation/multigrid/test_block_stepper.py

@@ -198,7 +198,7 @@ class CpuContracts(unittest.TestCase):
         backend._copy = backend._step = Mock()
         backend.boundary, backend._curved, backend._tables, backend._links = None, None, {}, 0
         backend._dst, backend._src = Mock(), Mock()
-        backend._block, backend._grid, backend._copy_grid = (32, 1, 1), (1, 1, 1), (1, 1)
+        backend._block, backend._grid = (32, 1, 1), (1, 1, 1)
         backend._allocations = [Mock(), Mock()]
         backend._current, backend._spare = backend._allocations
         return backend