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.

dora_ulite6/config.py

@@ -60,9 +60,10 @@ class MotionConfig:
     ready_timeout_s: float = 3.0
     home_timeout_s: float = 20.0
     tool_settle_s: float = 2.0
-    # The controller's own expiry for one jog command: the arm keeps moving at
-    # the commanded velocity until this runs out, so a sender that stops sending
-    # stops the arm.
+    # How long a jog stream may go quiet before this node stops the arm. The
+    # velocity is set on the controller once and held there indefinitely — see
+    # `ULite6.set_cartesian_velocity` — so nothing expires it but us: this is
+    # the deadman behind a sender that dies mid-jog, enforced by `_expire_jog`.
     velocity_command_ttl_s: float = 0.5
 
     @classmethod