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_block_stepper.py

@@ -159,11 +159,17 @@ class CpuContracts(unittest.TestCase):
             with self.assertRaisesRegex(ValueError, "launch limits"):
                 single(context=self.forbidden_context())
 
-    def test_param_layout_rejected_before_allocation(self):
+    def test_memory_and_param_layout_rejected_before_allocation(self):
         context = Mock(activate=nullcontext, sm_arch="sm_70")
+        with patch.object(cuda, "mem_get_info", return_value=(1024, 1 << 30)), \
+                patch.object(cuda, "Stream") as stream, patch.object(cuda, "mem_alloc") as alloc:
+            with self.assertRaisesRegex(MemoryError, "exceed free device memory"):
+                single(context=context, objects=[circle()])
+            stream.assert_not_called()
+            alloc.assert_not_called()
         module = Mock()
         module.get_global.return_value = (Mock(), 16)
-        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") as compile_kernel, \
                 patch.object(compiler, "cleanup_build_dir"), \
                 patch.object(compiler, "load_module", return_value=module), \