Fix periodic-seam cut-link addressing in map_cut_links

Source (write) and reverse-donor addresses are now the receiver's local
core address +/- (ey*local_nx + ex), i.e. the window image the next local
pull reads. block_cell preferred the in-window core image on a one-block
periodic axis, so BFL wrote a copy the pull never reads. Physical
neighbors are still formed only to reject non-periodic exteriors.
block_cell semantics unchanged; docstring notes the image ambiguity.
Adds an end-to-end synthetic seam test (x=0/x=nx-1/corner wraps) on
one-block-x, one-block-y and 1x1 periodic layouts.

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

src/CelerisLab/lbm/blocks.py

@@ -152,7 +152,9 @@ class BlockLayout2D:
     def block_cell(self, block, y, x):
         """Address of in-grid cell (y, x) in ``block``'s own storage, core or halo.
 
-        Periodic axes resolve to the image inside the block's halo window.
+        Periodic axes resolve to the image inside the block's halo window;
+        if two images fit, the core-side one is returned (use local offsets,
+        not this lookup, when a stencil must follow a specific image).
         Raises if the cell is not represented there or lies across an exterior.
         """
         by, bx = self.block
@@ -187,6 +189,7 @@ class BlockLayout2D:
         grid. Halo must be at least one cell (always true here).
         """
         ny, nx = self.shape
+        lx = self.local_shape[1]
         cells = int(np.prod(self.local_shape))
         rows, seen, writers = [], set(), set()
         for link in links:
@@ -197,16 +200,20 @@ class BlockLayout2D:
             y, x = divmod(r, nx)
             receiver = int(self.owner[y, x])
             block = receiver // cells
-            sy, sx = self._neighbor(y, x, D2Q9_EY[q], D2Q9_EX[q])
+            # Local stencil offsets from the core receiver: the window image the
+            # next local pull reads, even where a periodic seam wraps physically.
+            # Physical neighbors are formed only to reject non-periodic exteriors.
+            step = D2Q9_EY[q] * lx + D2Q9_EX[q]
+            self._neighbor(y, x, D2Q9_EY[q], D2Q9_EX[q])
             opp = q + 1 if q % 2 else q - 1
-            write = int(self.slot(self.block_cell(block, sy, sx), opp))
+            write = int(self.slot(receiver + step, opp))
             if write in writers:
                 raise ValueError("duplicate local boundary writer")
             writers.add(write)
             donor = NO_DONOR
             if float(link.q) < .5 and int(link.fallback) == 0:
-                dy, dx = self._neighbor(y, x, -D2Q9_EY[q], -D2Q9_EX[q])
-                donor = self.block_cell(block, dy, dx)
+                self._neighbor(y, x, -D2Q9_EY[q], -D2Q9_EX[q])
+                donor = receiver - step
             rows.append((block, receiver, donor, write))
         table = np.array(rows, dtype=np.uint64).reshape(-1, 4)
         return BlockCutLinks(_readonly(table[:, 0], np.uint32),