simulation: consume a zero-motion translation target; empty diagnostics shape

A pending target equal to the applied offset uploaded the same record, so
_run_moving saw no advance and never committed it: pending stayed latched,
blocking run(n > 1) and direct steps. Commit whenever a target was pending
(no_donor check and hold-record re-upload still only on advancing steps).
moving_diagnostics() without moving circles now returns (0, DIAG_FLOATS)
instead of the stale (0, 6). Tests for both.

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

src/CelerisLab/simulation.py

@@ -242,7 +242,8 @@ class Simulation:
         self._require_usable()
         tables = self.stepper.moving
         if tables is None:
-            return np.zeros((0, 6), np.float32)
+            from .body.motion import MovingTables
+            return np.zeros((0, MovingTables.DIAG_FLOATS), np.float32)
         self.stream.synchronize()
         return tables.download_diagnostics()
 
@@ -785,6 +786,7 @@ class Simulation:
         (docs/moving_bodies.md 4a, 5). Any failure leaves the Simulation unusable."""
         tables = self.stepper.moving
         circles = [(obj.motion, obj.obj_id) for obj in self._moving_bodies()]
+        pending = any(motion.pending is not None for motion, _ in circles)
         try:
             advancing = tables.upload_records(circles, stream)
             self.stepper._step(steps, action_gpu=self.bodies.action_gpu,
@@ -797,10 +799,12 @@ class Simulation:
                 no_donor = tables.download_diagnostics()[:, tables.DIAG.index("no_donor")]
                 if np.any(no_donor > 0):
                     raise RuntimeError("translating circle refill found no donor cell")
+            if pending:  # a target equal to the applied offset is consumed as a hold step
                 for motion, _ in circles:
                     motion.commit()
-                tables.upload_records(circles, stream)  # first hold: old = new = applied, u = 0
-                stream.synchronize()
+                if advancing:
+                    tables.upload_records(circles, stream)  # first hold: old = new = applied, u = 0
+                    stream.synchronize()
                 self._attach_moving()
         except BaseException as exc:
             for motion, _ in circles:  # device geometry/state unknown after any failure

tests/unit/test_moving_bodies.py

@@ -297,6 +297,26 @@ def test_run_one_advances_and_commits_after_sync(monkeypatch):
     assert ("step", 3, False) in writes and motion.applied == (0., .25)
 
 
+def test_zero_motion_target_is_consumed_by_run_one(monkeypatch):
+    sim, writes, motion = advancing_simulation(monkeypatch)
+    motion.set_target((0., 0.), (0., 0.))          # equal to applied: a hold target
+    sim._attach_moving()
+    assert sim.stepper.blocked == module.PENDING_BLOCKED
+    sim.run(1, stream=STREAM)
+    assert motion.pending is None and motion.applied == (0., 0.)
+    assert sim.stepper.blocked is None and ("step", 1, False) in writes
+    writes.clear()
+    sim.run(3, stream=STREAM)                      # no stale latch
+    assert ("step", 3, False) in writes
+
+
+def test_moving_diagnostics_empty_shape_matches_layout(monkeypatch):
+    from CelerisLab.body.motion import MovingTables
+    sim = make_simulation(monkeypatch)
+    sim._failed, sim.stepper = None, SimpleNamespace(moving=None)
+    assert sim.moving_diagnostics().shape == (0, MovingTables.DIAG_FLOATS)
+
+
 @pytest.mark.parametrize("failure", ("upload", "first_sync", "no_donor", "hold_upload",
                                      "hold_sync"))
 def test_failed_advance_fails_simulation(monkeypatch, failure):