fix jogging stepping by holding one velocity instead of re-arming

`set_cartesian_velocity` passed `duration=velocity_command_ttl_s` on every
message, so each one re-armed a timed segment and the controller re-planned
the same motion. On a real Lite6 that is not smooth travel: the arm moves in
steps. Reducing how often the velocity is sent does not help — re-arming four
times a second still steps — so it now uses `duration=0`, the SDK's "always
effective, will not stop automatically", and skips a velocity that has not
changed. One command stands until a different one replaces it.

That removes the controller-side expiry, which `_expire_jog` was relying on:
it flipped `_jogging` and reported `ready` without commanding anything,
because the controller's own timer had already stopped the arm. With the
velocity held indefinitely that would leave an arm travelling while the node
reported idle, so `_expire_jog` now queues the stop itself. It goes through
the job queue rather than the driver, since it runs on the reporter thread
and the control thread owns the driver.

`VELOCITY_COMMAND_TTL_S` is unchanged in value and meaning to a caller, but
it is now enforced here rather than by the controller.

Both halves are one commit on purpose: `duration=0` without the stop in
`_expire_jog` is an arm that never stops when its sender dies.

Adds tests/test_driver.py — the driver had no direct coverage, because
FakeArm replaces it everywhere else, which is why this was invisible to the
suite.

tests/test_lifecycle.py

@@ -205,8 +205,7 @@ def test_a_zero_velocity_jog_does_not_make_a_running_motion_look_idle(monkeypatc
 
 
 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.
+    # Nothing tells us a sender died, so the clock has to.
     control = build(monkeypatch, motion=MotionConfig(velocity_command_ttl_s=0.02))
     try:
         wait_for(lambda: READY in control.take_state_changes())
@@ -218,6 +217,22 @@ def test_a_sender_that_goes_quiet_stops_being_called_jogging(monkeypatch):
         control.shutdown()
 
 
+def test_a_sender_that_goes_quiet_actually_stops_the_arm(monkeypatch):
+    # The velocity is held by the controller indefinitely, so the expiry is the
+    # deadman itself and not a note about one. Reporting `ready` without
+    # commanding the stop would leave the arm travelling while the node said it
+    # was idle, which is worse than not expiring it at all.
+    control = build(monkeypatch, motion=MotionConfig(velocity_command_ttl_s=0.02))
+    try:
+        wait_for(lambda: READY in control.take_state_changes())
+        control.start_jog([20.0, 0, 0, 0, 0, 0])
+        wait_for(lambda: ("jog", [20.0, 0, 0, 0, 0, 0]) in control.arms[0].calls)
+
+        wait_for(lambda: ("jog", [0.0] * 6) in control.arms[0].calls)
+    finally:
+        control.shutdown()
+
+
 def test_a_jog_preempts_a_motion_that_is_running(monkeypatch):
     control = build(monkeypatch)
     control.arms[0].block = threading.Event()