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>

tests/unit/test_block_layout.py

@@ -39,6 +39,11 @@ def dense_pull(a, periodic):
 def blocked_pull(layout, blocked):
     """Exchange, then pull every core cell from its own block storage only."""
     layout.exchange(blocked)
+    return blocked_pull_core(layout, blocked)
+
+
+def blocked_pull_core(layout, blocked):
+    """Pull core cells with no halo copy (a copy would undo BFL halo writes)."""
     out = np.full_like(blocked, np.nan)
     (by, bx), h = layout.block, layout.halo
     for q in range(NQ):
@@ -218,15 +223,30 @@ class BlockLayoutTests(unittest.TestCase):
         layout = BlockLayout2D.build(16, 16, 8, 8)
         blocked = layout.to_blocked(a)  # gather halos
         blocked_bfl(layout, blocked, links, layout.map_cut_links(links))
-        # Pull core cells without another halo copy (it would undo halo writes).
-        out = np.full_like(blocked, np.nan)
-        (by, bx), h = layout.block, layout.halo
-        for q in range(NQ):
-            ey, ex = D2Q9_EY[q], D2Q9_EX[q]
-            out[:, q, h:h + by, h:h + bx] = blocked[:, q, h - ey:h - ey + by, h - ex:h - ex + bx]
-        got = layout.to_dense(out)
+        got = layout.to_dense(blocked_pull_core(layout, blocked))
         np.testing.assert_array_equal(got[:, ~solid], expected[:, ~solid])
 
+    def test_periodic_seam_links_follow_the_pulled_halo_image(self):
+        # Receivers on x=0 / x=nx-1 / a corner whose sources wrap physically.
+        links = [CutLink(fluid_idx=1 * 6 + 0, dir=2, q=.6, body_id=0),
+                 CutLink(fluid_idx=2 * 6 + 0, dir=2, q=.3, body_id=0),
+                 CutLink(fluid_idx=3 * 6 + 5, dir=1, q=.3, body_id=0),
+                 CutLink(fluid_idx=0 * 6 + 0, dir=6, q=.4, body_id=0)]
+        sources = np.zeros((8, 6), dtype=bool)
+        for y, x in ((1, 5), (2, 5), (3, 0), (7, 5)):
+            sources[y, x] = True
+        a = tags(8, 6) / 7.0
+        expected = dense_pull(dense_bfl(a, links), (True, True))
+        # One block along x, along y, along both (periodic self-images).
+        for by, bx in ((4, 6), (8, 3), (8, 6)):
+            with self.subTest(block=(by, bx)):
+                layout = BlockLayout2D.build(8, 6, by, bx)
+                mapped = layout.map_cut_links(links)
+                blocked = layout.to_blocked(a)
+                blocked_bfl(layout, blocked, links, mapped)
+                got = layout.to_dense(blocked_pull_core(layout, blocked))
+                np.testing.assert_array_equal(got[:, ~sources], expected[:, ~sources])
+
     def test_cut_link_rejections(self):
         layout = BlockLayout2D.build(16, 16, 8, 8, periodic=(False, False))
         link = CutLink(fluid_idx=5 * 16 + 5, dir=1, q=.6, body_id=0)