body: host-only translating-circle state (S1, no kernels)

MovingCircle keeps the applied offset and an optional pending target apart,
validates targets against the applied offset (box, Euclidean 0.5-cell step,
speed, no sliding hold) and commits only after upload, step and sync succeed;
any failure after the upload attempt is fail-closed. Simulation.add_body
(translation=...) declares one; moving circles are excluded from the packed
static links, unsupported modes and swept-band placement are rejected at
initialize, and every positive-step run raises before any device write until
the moving kernels exist. Host tests include a refill-donor enumeration.

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

tests/unit/test_moving_bodies.py

@@ -0,0 +1,131 @@
+"""Translating-circle host contract, S1 (docs/moving_bodies.md 4a, 5, 7).
+
+Host doubles only: no kernels exist yet, so every positive-step run with a
+moving body must be rejected before any device write.
+"""
+from types import SimpleNamespace
+
+import pytest
+
+from CelerisLab.body.motion import (MAX_STEP, R_MIN, MovingCircle, Translation,
+                                    enumerate_donor_coverage)
+
+BAD = (float("nan"), float("inf"))
+
+
+def circle(box=(-2., 2., -3., 3.), speed=.1):
+    return MovingCircle((50., 40.), 10., Translation(box, speed))
+
+
+def test_translation_and_radius_validation():
+    for box in ((0., 1., -1., 1.)[:3], (.5, 1., -1., 1.), (-1., 1., BAD[0], 1.)):
+        with pytest.raises(ValueError):
+            Translation(box, .1)
+    for speed in (0., MAX_STEP + 1e-9) + BAD:
+        with pytest.raises(ValueError):
+            Translation((0., 0., -1., 1.), speed)
+    with pytest.raises(ValueError):
+        MovingCircle((0., 0.), R_MIN - .1, Translation((0., 0., -1., 1.), .1))
+
+
+def test_targets_validate_against_applied_and_last_wins():
+    c = circle()
+    c.set_target((0., .3), (0., .05))
+    c.set_target((0., .5), (0., .05))  # second setter replaces, checked vs applied (0, 0)
+    assert c.pending == ((0., .5), (0., .05))
+    for offset, velocity in (((0., .51), (0., 0.)),           # Euclidean bound
+                             ((.36, .36), (0., 0.)),          # |dc|_2 = .509
+                             ((0., 3.1), (0., 0.)),           # outside box
+                             ((0., .1), (0., .11)),           # speed bound
+                             ((0., 0.), (0., .01)),           # hold with sliding velocity
+                             ((BAD[0], 0.), (0., 0.)), ((0., 0.), (0., BAD[1]))):
+        with pytest.raises(ValueError):
+            c.set_target(offset, velocity)
+        assert c.pending == ((0., .5), (0., .05))  # invalid replacement keeps the old target
+    c.set_target((.35, .35), (0., 0.))  # |dc|_2 = .495 inside the bound
+    assert c.applied == (0., 0.)
+
+
+def test_record_commit_and_hold():
+    c = circle()
+    assert c.record() == ((50., 40.), (50., 40.), (0., 0.))  # hold: no translation velocity
+    c.set_target((0., .25), (0., .05))
+    assert c.record() == ((50., 40.), (50., 40.25), (0., .05))
+    uploads = []
+    c.advance(uploads.append, lambda: None, lambda: None)
+    assert uploads == [((50., 40.), (50., 40.25), (0., .05))]
+    assert c.applied == (0., .25) and c.pending is None  # offset-only commit
+    c.advance(uploads.append, lambda: None, lambda: None)  # no new target: hold step
+    assert uploads[-1] == ((50., 40.25), (50., 40.25), (0., 0.)) and c.applied == (0., .25)
+    c.set_target((0., 0.), (0., -.05))  # return to zero offset is a real target
+    assert c.pending == ((0., 0.), (0., -.05))
+    c.advance(uploads.append, lambda: None, lambda: None)
+    assert c.applied == (0., 0.) and c.pending is None
+
+
+@pytest.mark.parametrize("stage", ("upload", "step", "synchronize"))
+def test_failure_after_upload_attempt_is_fail_closed(stage):
+    c = circle()
+    c.set_target((0., .25), (0., .05))
+
+    def call(name):
+        def run(*_):
+            if name == stage:
+                raise RuntimeError(f"{name} failed")
+        return run
+    with pytest.raises(RuntimeError, match=f"{stage} failed"):
+        c.advance(call("upload"), call("step"), call("synchronize"))
+    assert c.failed and c.applied == (0., 0.)
+    for later in (c.record, lambda: c.set_target((0., .1), (0., 0.)),
+                  lambda: c.advance(*(lambda *_: None,) * 3)):
+        with pytest.raises(RuntimeError, match="unknown"):
+            later()
+
+
+def test_band_covers_offset_box_with_margin():
+    assert circle().band() == (50 - 2 - 10 - 2, 50 + 2 + 10 + 2, 40 - 3 - 10 - 2, 40 + 3 + 10 + 2)
+
+
+@pytest.mark.parametrize("radius", (R_MIN, 4.3, 7.5, 12.0))
+def test_refill_donor_sets_nonempty_within_bounds(radius):
+    # Evidence for R >= R_MIN and |dc|_2 <= MAX_STEP on a fixed sample, not a proof.
+    assert enumerate_donor_coverage(radius) == 0
+
+
+pytest.importorskip("pycuda.driver")
+from CelerisLab import simulation as module  # noqa: E402
+from tests.unit.test_simulation_lifecycle import make_simulation  # noqa: E402
+
+
+def moving_simulation(monkeypatch, *, moving=True):
+    sim = make_simulation(monkeypatch)
+    obj = SimpleNamespace(motion=circle() if moving else None, obj_id=0,
+                          center=(50., 40.), radius=10.)
+    writes = []
+    sim._failed, sim._initialized = None, True
+    sim.bodies = SimpleNamespace(objects=[obj], count=1, jets=[],
+                                 has_pending_edit=lambda: False,
+                                 _upload_action_async=lambda s: writes.append("action"),
+                                 zero_obs_async=lambda s: writes.append("obs"))
+    sim.stepper = SimpleNamespace(step=lambda *a, **k: writes.append("step"))
+    return sim, writes
+
+
+def test_positive_steps_rejected_before_device_writes(monkeypatch):
+    sim, writes = moving_simulation(monkeypatch)
+    sim.bodies.objects[0].motion.set_target((0., .1), (0., .01))
+    for call in (lambda: sim.run(1), lambda: sim.run(5), lambda: sim.step(),
+                 lambda: sim.run(3, checkpoint_interval=1)):
+        with pytest.raises(RuntimeError, match="moving kernels"):
+            call()
+    with pytest.raises(RuntimeError, match="moving kernels"):  # nominal at rest as well
+        sim.bodies.objects[0].motion.pending = None
+        sim.run(1)
+    assert writes == [] and sim.bodies.objects[0].motion.applied == (0., 0.)
+
+
+def test_moving_body_rejects_topology_and_checkpoint(monkeypatch):
+    sim, _ = moving_simulation(monkeypatch)
+    for call in (sim.sync_bodies, sim.save_checkpoint, lambda: sim.load_checkpoint("x.h5")):
+        with pytest.raises(ValueError, match="translating circles"):
+            call()