Release the compiled module on close

close() now clears _module (initialized to None) inside the active
context together with the function references; lifecycle test asserts
it is retained on failed activation and cleared after close.

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

tests/validation/multigrid/test_block_stepper.py

@@ -114,6 +114,7 @@ class CpuContracts(unittest.TestCase):
         backend._lock = RLock()
         backend._status = "ready"
         backend._stream = Mock()
+        backend._module = Mock()
         backend._copy = backend._step = Mock()
         backend._dst, backend._src = Mock(), Mock()
         backend._block, backend._grid, backend._copy_grid = (32, 1, 1), (1, 1, 1), (1, 1)
@@ -155,7 +156,7 @@ class CpuContracts(unittest.TestCase):
 
     def test_activation_failure_retains_resources_for_retry(self):
         backend = self.fake_backend()
-        allocations, stream = backend._allocations[:], backend._stream
+        allocations, stream, module = backend._allocations[:], backend._stream, backend._module
         unavailable = MagicMock()
         unavailable.__enter__.side_effect = RuntimeError("activation failed")
         backend.ctx.activate = Mock(side_effect=[unavailable, nullcontext()])
@@ -164,8 +165,12 @@ class CpuContracts(unittest.TestCase):
         self.assertEqual(backend._status, "failed")
         self.assertEqual(backend._allocations, allocations)
         self.assertIs(backend._stream, stream)
+        self.assertIs(backend._module, module)
         backend.close()
         self.assertEqual(backend._status, "closed")
+        self.assertIsNone(backend._module)
+        self.assertIsNone(backend._copy)
+        self.assertIsNone(backend._step)
         for allocation in allocations:
             allocation.free.assert_called_once()