BlockLayout2D: copy caller-supplied origins

An int64 C-contiguous origins array was kept by reference (and frozen in
the caller's hands), so mutating it later changed layout.origins and
block_cell without changing owner/halos. Copy before marking read-only;
regression checks origins, owner and block_cell invariance.

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

src/CelerisLab/lbm/blocks.py

@@ -144,7 +144,7 @@ class BlockLayout2D:
             jy, jx = np.divmod(np.arange(nby * nbx), nbx)
             origins = np.stack([jy * by, jx * bx], axis=1)
         selected = _selected_blocks(origins, ny, nx, by, bx)
-        origins = np.asarray(origins, dtype=np.int64).reshape(-1, 2)
+        origins = np.array(origins, dtype=np.int64).reshape(-1, 2)  # own copy, never caller storage
         if (len(origins) + 1) * NQ * cells > np.iinfo(np.int64).max // 2:
             raise ValueError("blocked storage exceeds 64-bit indexing")
 

tests/unit/test_block_layout.py

@@ -411,6 +411,17 @@ class SparseBlockLayoutTests(unittest.TestCase):
         np.testing.assert_array_equal(stored[layout.owner.ravel()[owned].astype(np.intp)], flags[owned])
         self.assertEqual(np.count_nonzero(stored), owned.sum())
 
+    def test_caller_origins_are_copied(self):
+        origins = np.array([[0, 0], [0, 4]], dtype=np.int64)
+        layout = BlockLayout2D.build(6, 8, 3, 4, origins=origins)
+        owner, address = layout.owner.copy(), layout.block_cell(0, 0, 0)
+        self.assertTrue(origins.flags.writeable)  # caller array is not frozen
+        origins[0] = [3, 0]
+        np.testing.assert_array_equal(layout.origins, [[0, 0], [0, 4]])
+        np.testing.assert_array_equal(layout.owner, owner)
+        self.assertEqual(layout.block_cell(0, 0, 0), address)
+        self.assertEqual(address, layout.owner[0, 0])
+
     def test_invalid_origins_rejected(self):
         for origins in ([(0, 4), (0, 0)],          # not row-major
                         [(3, 0), (0, 4)],          # not row-major across rows