fix: validate passive boundary ownership and expose Jet exchange

tests/unit/test_jets.py

@@ -188,6 +188,55 @@ def jet_case(device, jets):
     return field, bodies
 
 
+def test_public_surface_exchange_for_jet_preserves_aperture_at_zero_and_reverse(device):
+    from CelerisLab.simulation import Simulation
+    jet = Jet(Sector(CENTER, 0., .6, 3.), Radial(CENTER, inward=False), "quartic", 2e-3)
+    field, bodies = jet_case(device, [jet])
+    sim = SimpleNamespace(bodies=bodies, field=field, _initialized=True,
+                          stream=SimpleNamespace(synchronize=device.synchronize))
+    cl = field.curved
+    assert bodies.get(0).surface_actuation is None
+    aperture, velocity = cl.aperture, cl.surface_velocity.copy()
+    assert aperture.any() and (~aperture).any()
+    # Synthetic per-link exchanges test readback/splitting, not kernel physics.
+    values = np.arange(cl.count * 3, dtype=np.float32).reshape(-1, 3)
+    device.htod(cl.link_exchange_gpu, values)
+    for amplitude in (2e-3, 0., -1e-3):
+        Simulation.set_jet_amplitude(sim, 0, amplitude)
+        result = Simulation.read_surface_exchange(sim, 0)
+        assert cl.aperture is aperture
+        np.testing.assert_array_equal(aperture, cl.jet_id >= 0)
+        np.testing.assert_allclose(cl.surface_velocity, velocity * (amplitude / 2e-3))
+        for name, mask in (("wall", ~aperture), ("aperture", aperture)):
+            np.testing.assert_array_equal(result[name], values[mask].sum(axis=0, dtype=np.float64))
+
+
+def test_public_surface_exchange_keeps_legacy_and_passive_guards(device):
+    from CelerisLab.simulation import Simulation
+    field = make_field(device)
+    bodies = manager(field, [circle(SurfaceActuation(0., .6, .002), radius=2.)])
+    bodies.sync_to_gpu(field)
+    sim = SimpleNamespace(bodies=bodies, field=field,
+                          stream=SimpleNamespace(synchronize=device.synchronize))
+    for value in Simulation.read_surface_exchange(sim, 0).values():
+        np.testing.assert_array_equal(value, 0.)
+    field.curved.free_gpu_columns()
+    with pytest.raises(RuntimeError, match="diagnostic is allocated"):
+        Simulation.read_surface_exchange(sim, 0)
+
+    # An exchange buffer elsewhere in the scene must not admit a passive body.
+    for jets in ([], [Jet(Sector(CENTER, 0., .6, 3.), Radial(CENTER))]):
+        field, bodies = jet_case(device, jets)
+        passive = bodies.add(circle(center=(2., 3.), radius=.6))
+        bodies.sync_to_gpu(field)
+        sim.bodies, sim.field = bodies, field
+        with pytest.raises(ValueError, match="Body has no surface actuation"):
+            Simulation.read_surface_exchange(sim, passive)
+        if not jets:
+            with pytest.raises(ValueError, match="Body has no surface actuation"):
+                Simulation.read_surface_exchange(sim, 0)
+
+
 def test_manager_passes_true_hits_and_reuploads_only_velocity(device):
     # CPU memory double only: checks host/device bookkeeping, not GPU numerics.
     jet = Jet(Sector(CENTER, 0., .6, 3.), Radial(CENTER, inward=False), "quartic", 2e-3)