BlockLayout2D.build: check capacity before generating regular origins

c397e08 moved the 64-bit capacity check after np.arange(nby*nbx), so an
over-limit regular layout allocated before rejecting. Check arithmetic
block count (regular) or validated selected count (sparse) first; mock
regression asserts no np.arange before the ValueError.

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

tests/unit/test_block_layout.py

@@ -1,6 +1,7 @@
 """CPU block layout contracts: ownership, halo pairs, packing and tagged pull."""
 import sys
 import unittest
+from unittest import mock
 from types import SimpleNamespace
 
 import numpy as np
@@ -280,6 +281,12 @@ class BlockLayoutTests(unittest.TestCase):
         lx = layout.local_shape[1]
         self.assertEqual(int(mapped.write[0]), int(layout.slot(3 * lx + 0, 1)))
 
+    def test_capacity_rejected_before_allocation(self):
+        fail = AssertionError("grid-sized allocation before the capacity check")
+        with mock.patch.object(np, "arange", side_effect=fail):
+            with self.assertRaises(ValueError):
+                BlockLayout2D.build(2 ** 30, 2 ** 30, 1, 1)
+
     def test_tables_are_readonly(self):
         layout = BlockLayout2D.build(6, 8, 3, 4)
         for array in (layout.origins, layout.owner, layout.halo_dst, layout.halo_src, layout.halo_kind):