Test: jet ownership is set membership, not row adjacency

Synthetic rows: one jet over non-contiguous rows of one body is accepted;
the same rows with a second body id are rejected (Grok case 2).

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

tests/unit/test_jets.py

@@ -346,3 +346,30 @@ def test_separate_jets_on_separate_bodies_accepted(device):
     for j in (0, 1):
         assert len(np.unique(cl.body_id[cl.jet_id == j])) == 1
     assert cl.body_id[cl.jet_id == 0][0] != cl.body_id[cl.jet_id == 1][0]
+
+
+def test_one_body_with_noncontiguous_rows_accepted_and_mixed_rejected():
+    # Synthetic rows: ownership is set membership over body_id, not row adjacency.
+    from CelerisLab.body.geometry.circle import CircleGeometry
+    from CelerisLab.body.objects import SimObject
+    from CelerisLab.config import LBMConfig
+    from CelerisLab.lbm.descriptors import FLUID
+    cfg = LBMConfig(nx=12, ny=10, viscosity=.1)
+    cfg.validate()
+    objs = [SimObject(i, geometry=CircleGeometry(3. + 5*i, 5., 1.), center=(3. + 5*i, 5.), radius=1.)
+            for i in (0, 1)]
+    flags = np.full(cfg.nx*cfg.ny, FLUID, np.uint16)
+    fluid_idx = np.array([10, 20, 30], np.uint32)
+    dirs = np.array([1, 1, 1], np.uint8)
+    zeros = np.zeros(3, np.float32)
+    hits = np.array([[0., 0.], [5., 5.], [0., 1.]])
+    jet = Jet(Tube((0., -.5), math.pi/2, 2., .5), Parallel(0.))  # selects rows 0 and 2
+    for body_id, ok in ((np.array([0, 1, 0], np.int32), True), (np.array([0, 1, 1], np.int32), False)):
+        call = lambda: surface_link_velocity(objs, cfg, flags, fluid_idx, dirs, body_id,
+                                             zeros, zeros, jets=[jet], hits=hits)
+        if ok:
+            _, aperture = call()
+            assert aperture.tolist() == [True, False, True]
+        else:
+            with pytest.raises(ValueError, match="one jet may drive only one body"):
+                call()