refinement: free-memory preflight for every level before any device work

Restores the PeriodicBlockStepper safeguard dropped in 8e1feff, for all
FixedPatchStepper levels: both G buffers, halo pairs and boundary columns
of every level are compared with cuda.mem_get_info() before the stream,
any module or any buffer exists, raising MemoryError('level buffers exceed
free device memory'). Transfer tables are not counted (lower bound).

Tests: the single-level MemoryError case is back beside the param-layout
check (no stream, no allocation); host test_single_level covers depth 1
and depth 2 (no stream, build or allocation) and the wait failure after a
good step (count stays 1). Driver-mocking fixtures report ample free memory.

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

tests/validation/multigrid/test_refinement_three_level.py

@@ -56,7 +56,7 @@ def mocked_child():
     context = Mock(activate=nullcontext, sm_arch="sm_70")
     module = Mock(get_function=Mock(side_effect=lambda name: Mock(name=name)))
     module.get_global.return_value = (Mock(), 20)
-    with patch.object(cuda, "Stream"), patch.object(compiler, "generate_config"), \
+    with patch.object(cuda, "Stream"), patch.object(cuda, "mem_get_info", return_value=(1 << 40, 1 << 40)), patch.object(compiler, "generate_config"), \
             patch.object(compiler, "compile_kernel"), patch.object(compiler, "cleanup_build_dir"), \
             patch.object(compiler, "load_module", return_value=module), \
             patch.object(cuda, "mem_alloc", side_effect=lambda size: Mock()), \