test: adopt native-validated 24-target probe layout

Horizontal y=56 and vertical x=38 through box 1, normal coordinates
[12.5..26.125] (root128 physical). Nearest-ACTIVE mapping with
deterministic tie rule (distance, finer level, smaller j, smaller i),
duplicate-cell rejection, stored centers and displacements. Replaces my
earlier offsets, which mapped two target pairs to the same U128 cell.
U512 spacing 128/512 and root step r -> U512 step 4r unchanged.
Tests: 24 unique cells per case, nested 3 L0 + 5 L1 + 4 L2 per transect,
U128 max displacement 0.625.

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

tests/validation/taylor_green/test_shear_wave.py

@@ -977,11 +977,17 @@ class ProbeTraceTests(unittest.TestCase):
         u512 = sp.select([np.ones((512, 512), bool)], 512)
         self.assertTrue(all(p["level"] == 0 for p in u128 + u512))
         for case in (nested, twin, u128, u512):
+            self.assertEqual(len({(p["level"], p["i"], p["j"]) for p in case}), 24)  # unique
             for p in case:
-                # The stored center is the actual cell center containing the point (root units).
-                h = 1.0 / (2 ** p["level"]) if case is not u512 else 0.25
-                self.assertLessEqual(abs(p["center"][0] - p["point"][0]), h / 2)
-                self.assertLessEqual(abs(p["center"][1] - p["point"][1]), h / 2)
+                dx, dy = p["displacement"]
+                self.assertEqual([p["center"][0] - p["point"][0], p["center"][1] - p["point"][1]],
+                                 [dx, dy])
+        self.assertEqual(max(math.hypot(*p["displacement"]) for p in u128), 0.625)
+        self.assertLessEqual(max(math.hypot(*p["displacement"]) for p in u512), 0.25 * 0.7072)
+        # Nested: 3 L0 + 5 L1 + 4 L2 per transect, as checked by native review.
+        for name in sp.TRANSECTS:
+            levels = [p["level"] for p in nested if p["transect"] == name]
+            self.assertEqual([levels.count(v) for v in (0, 1, 2)], [3, 5, 4])
         # Different grids give different centers: no pretended collocation.
         self.assertNotEqual(u128[3]["center"], u512[3]["center"])
         self.assertEqual(sp.case_steps(512)[:3], [0, 4, 8])