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>

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