body: stage motion-record uploads in an owned page-locked buffer

memcpy_htod_async needs page-locked host memory. MovingTables allocates one
pinned FP64 buffer at construction (manager.py action pattern) and rewrites
it only when the records change; every upload is followed by a stream sync
in Simulation._run_moving before the next one, so no copy is in flight when
it is reused. The advance test fixture now binds the real LBMStepper.step.

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

src/CelerisLab/body/motion.py

@@ -197,6 +197,8 @@ class MovingTables:
         self.geom = list(geom)  # host copy of the uploaded records
         self.band_gpu = cuda.to_device(np.asarray(band, np.int32))
         self.geom_gpu = cuda.to_device(np.asarray(geom, np.float64))
+        # Owned page-locked staging for async record uploads (allocated once).
+        self._geom_pinned = cuda.pagelocked_empty(len(geom), np.float64)
         self.diag_gpu = cuda.to_device(np.zeros(self.n_bodies * self.DIAG_FLOATS, np.float32))
         self.slot_debug_gpu = None  # optional (q, fallback, rx, ry) per slot for parity tests
 
@@ -224,13 +226,18 @@ class MovingTables:
         return [*c_old, *c_new, motion.radius, *u]
 
     def upload_records(self, circles, stream) -> bool:
-        """Upload the records when they differ from the device copy; True if advancing."""
+        """Enqueue the records when they differ from the device copy; True if advancing.
-        import numpy as np
+
+        The pinned staging buffer is reused: callers synchronize ``stream`` after every
+        upload before the next one (``Simulation._run_moving``), so no copy is in flight
+        when it is rewritten. ``geom`` is the enqueued state; after a failed copy the
+        owner is failed and the cache is never trusted again.
+        """
         import pycuda.driver as cuda
         geom = [v for motion, _ in circles for v in self.record(motion)]
         if geom != self.geom:
-            self._staged = np.asarray(geom, np.float64)  # keep alive until the copy completes
+            self._geom_pinned[:] = geom
-            cuda.memcpy_htod_async(self.geom_gpu, self._staged, stream)
+            cuda.memcpy_htod_async(self.geom_gpu, self._geom_pinned, stream)
             self.geom = geom
         g = self.GEOM_DOUBLES
         return any(geom[i:i + 2] != geom[i + 2:i + 4] for i in range(0, len(geom), g))

tests/unit/test_moving_bodies.py

@@ -272,6 +272,8 @@ def advancing_simulation(monkeypatch, **tables):
     sim.stepper = SimpleNamespace(
         moving=Tables(writes, **tables), blocked=None,
         _step=lambda n, advance=False, **k: writes.append(("step", n, advance)))
+    from CelerisLab.lbm.stepper import LBMStepper
+    sim.stepper.step = LBMStepper.step.__get__(sim.stepper)  # real public entry
     sim._moving_tables = sim.stepper.moving
     return sim, writes, sim.bodies.objects[0].motion