Test: negative-radius regression at integer center (16, 12)

At the half-integer center the -0.1/-0.5 circles were already rejected
as unresolved, so the case did not exercise the finite/positive-radius
guard; an integer center makes the squared radius mark one solid cell.

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

tests/validation/multigrid/test_block_stepper.py

@@ -134,7 +134,7 @@ 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),
+        for center, radius in (((16.0, 12.0), -.1), ((16.0, 12.0), -.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(),