fix zero-velocity jog reporting a running motion as idle

start_jog restored READY for any zero velocity, without checking whether a
motion owned the arm. A teleop source streaming zero-velocity ticks during a
move published moving -> ready mid-motion, and the closing guard in _run_motion
then saw READY rather than MOVING and never republished, so a caller watching
robot_state concluded the motion had finished early.

Route every "the arm has stopped" transition through _idle_unless_busy, which
reports READY only when no motion is running, no jog is driving the arm, and the
state has not already left MOVING for unavailable or recovering.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_lifecycle.py

@@ -130,6 +130,29 @@ def test_a_zero_velocity_jog_puts_the_arm_back_to_ready(monkeypatch):
         control.shutdown()
 
 
+def test_a_zero_velocity_jog_does_not_make_a_running_motion_look_idle(monkeypatch):
+    # A teleop source streaming zero velocity says nothing about a motion that
+    # owns the arm, and must not answer for it: an arm that is travelling must
+    # never look idle, and a `ready` here would also stop the motion's own end
+    # from ever being published.
+    control = build(monkeypatch)
+    control.arms[0].block = threading.Event()
+    try:
+        wait_for(lambda: READY in control.take_state_changes())
+        control.submit(ServiceCall(request_id="m1", row={"service": "home"}))
+        wait_for(lambda: MOVING in control.take_state_changes())
+
+        for _ in range(5):
+            control.start_jog([0.0] * 6)
+
+        assert control.take_state_changes() == []
+
+        control.arms[0].block.set()
+        wait_for(lambda: READY in control.take_state_changes())
+    finally:
+        control.shutdown()
+
+
 def test_a_sender_that_goes_quiet_stops_being_called_jogging(monkeypatch):
     # The controller expires the command on its own; nothing tells us, so the
     # clock has to.