Reject nonfinite or nonpositive circle geometry; tuple-normalize objects

_static_circles now checks finite center/radius and radius > 0 before
build_flag_mask (a negative radius squared to a one-cell solid), matching
prepare_fine_boundary. objects is converted to a tuple before the
emptiness test so an empty iterable takes the body-free path.
CPU tests: negative/NaN radius and nonfinite center rejected before CUDA;
empty iterator skips circle preparation.

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

tests/validation/multigrid/test_block_stepper.py

@@ -134,6 +134,15 @@ class CpuContracts(unittest.TestCase):
         for objects in ([moving], [circle((1.5, 11.5), 2.0)], [circle(), circle()]):
             with self.assertRaises(ValueError):
                 PeriodicBlockStepper(config(), layout, context=self.forbidden_context(), objects=objects)
+        for center, radius in (((15.5, 11.5), -.1), ((15.5, 11.5), -.5), ((15.5, 11.5), np.nan),
+                               ((np.inf, 11.5), 4.3), ((15.5, np.nan), 4.3)):
+            with self.subTest(center=center, radius=radius), self.assertRaises(ValueError):
+                PeriodicBlockStepper(config(), layout, context=self.forbidden_context(),
+                                     objects=[circle(center, radius)])
+        with patch.object(implementation, "_static_circles") as prepare:
+            with self.assertRaisesRegex(AssertionError, "CUDA reached"):  # body-free path
+                PeriodicBlockStepper(config(), layout, context=self.forbidden_context(), objects=iter(()))
+            prepare.assert_not_called()
         with patch.object(CircleGeometry, "build_cut_links",
                           lambda geometry, *a, **k: build_links(geometry, *a, **k)[1:]):
             with self.assertRaisesRegex(ValueError, "cover every fluid pull"):