kernels (K9): blocks.cu becomes an include-only entry; kernels under step/

blocks.cu keeps its header, includes and #error guards and then includes
the kernels, as kernel_v2.cu does:
- step/block_transfer.cu: BlockHaloCopy, BlockExplode,
  BlockExplodeProjected, BlockCoalesce
- step/block_boundary.cu: BlockCurvedBoundary, BlockAccumulateExchange
- step/block_step.cu: BlockStep (rejects BLOCK_ROOT_BC == 2)
- step/block_channel.cu: unchanged (mode 2 only)
Kernel text moved verbatim (checked against the previous blocks.cu) and in
the same definition order, so the translation unit is the same sequence of
kernels. The two source-reading tests follow the move:
test_projected_explosion reads step/block_transfer.cu, test_refinement_mrt
reads blocks.cu + step/block_step.cu; their assertions are unchanged.

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

tests/unit/test_projected_explosion.py

@@ -162,7 +162,7 @@ class ProjectedKernelTests(unittest.TestCase):
         targets = child.owner.ravel()[p.ghost_fine]
         donors = parent.owner.ravel()[p.ghost_coarse]
         source = (Path(__file__).resolve().parents[2] /
-                  "src/CelerisLab/lbm/kernels/blocks.cu").read_text()
+                  "src/CelerisLab/lbm/kernels/step/block_transfer.cu").read_text()
         kernel = source.split('extern "C" __global__ void BlockExplodeProjected(',1)[1]
         kernel = 'void BlockExplodeProjected(' + kernel.split('// Targets are receiver-local SOURCE slots',1)[0]
         def array(name, values, kind):