stepper: bind moving-circle kernels only for D2Q9 double buffer

Both kernels are compiled under DIM == 2, but LBMStepper requested them for
every double-buffer module, so stationary D3Q19 double-buffer construction
would fail at get_function. Moving circles already reject D3Q19 and EsoPull
at initialize. Stale latch comment updated. Parametrized CPU test with a
module double that lacks the symbols.

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

src/CelerisLab/lbm/stepper.py

@@ -21,7 +21,7 @@ class LBMStepper:
 
         self._esopull = (cfg.streaming == "esopull")
         # Set by the owner when a step would be wrong (translating circles
-        # excluded from the packed links before the moving kernels exist).
+        # without attached moving tables, a pending target, or a failed step).
         self.blocked: str | None = None
         self.moving = None  # MovingTables attached by the owner (translating circles)
 
@@ -39,8 +39,10 @@ class LBMStepper:
         self.apply_esopull_curved_fn = module.get_function("ApplyEsoPullCurvedKernel")
         self.sensor_fn = module.get_function("SensorKernel")
         self.force_region_fn = module.get_function("ForceRegionKernel")
-        self.moving_fn = None if self._esopull else module.get_function("MovingCircleLinks")
+        # Moving-circle kernels exist only in D2Q9 double-buffer modules.
-        self.moving_cells_fn = None if self._esopull else module.get_function("MovingCircleCells")
+        moving = not self._esopull and cfg.is_d2q9
+        self.moving_fn = module.get_function("MovingCircleLinks") if moving else None
+        self.moving_cells_fn = module.get_function("MovingCircleCells") if moving else None
 
         tpb = cfg.threads_per_block
         self.block = (tpb, 1, 1)

tests/unit/test_moving_bodies.py

@@ -310,6 +310,24 @@ def test_zero_motion_target_is_consumed_by_run_one(monkeypatch):
     assert ("step", 3, False) in writes
 
 
+@pytest.mark.parametrize("streaming,d2q9,bound", [("double_buffer", True, True),
+                                                  ("double_buffer", False, False),
+                                                  ("esopull", True, False)])
+def test_moving_kernels_bound_only_for_d2q9_double_buffer(monkeypatch, streaming, d2q9, bound):
+    from CelerisLab.lbm import stepper as stepper_module
+    monkeypatch.setattr(stepper_module.cuda, "mem_alloc", lambda n: object())
+    moving = {"MovingCircleLinks", "MovingCircleCells"}
+
+    def get_function(name):
+        if name in moving and not (d2q9 and streaming == "double_buffer"):
+            raise KeyError(name)                 # absent from D3Q19 / EsoPull modules
+        return lambda *a, **k: None
+    cfg = SimpleNamespace(streaming=streaming, threads_per_block=128, is_d2q9=d2q9,
+                          nx=16, ny=8, nz=4, nq=9 if d2q9 else 19)
+    stepper = stepper_module.LBMStepper(SimpleNamespace(), SimpleNamespace(get_function=get_function), cfg)
+    assert (stepper.moving_fn is not None) == bound and (stepper.moving_cells_fn is not None) == bound
+
+
 def test_moving_diagnostics_empty_shape_matches_layout(monkeypatch):
     from CelerisLab.body.motion import MovingTables
     sim = make_simulation(monkeypatch)