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>

src/CelerisLab/lbm/blocks.py

@@ -140,13 +140,17 @@ class BlockLayout2D:
         nby, nbx = ny // by, nx // bx
         ly, lx = by + 2 * halo, bx + 2 * halo
         cells = ly * lx
-        if origins is None:
-            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.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:
+        if origins is not None:
+            selected = _selected_blocks(origins, ny, nx, by, bx)
+            origins = np.array(origins, dtype=np.int64).reshape(-1, 2)  # own copy, never caller storage
+        # Capacity from arithmetic before any grid-sized allocation; hole =
+        # nblocks * cells is below this bound too.
+        if (nby * nbx if origins is None else len(origins)) * NQ * cells > np.iinfo(np.int64).max // 2:
             raise ValueError("blocked storage exceeds 64-bit indexing")
+        if origins is None:
+            selected = np.arange(nby * nbx)
+            jy, jx = np.divmod(selected, nbx)
+            origins = np.stack([jy * by, jx * bx], axis=1)
 
         # Grid block -> stored block, or len(origins) for an absent block, so
         # absent cells land exactly on hole = nblocks * cells.