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>
