read the gamepad from dora-joy and publish twist and command

The pad moves to dora-joy, so this node keeps no hardware dependency and the
pad becomes reusable. It arrives on `joy` as a plain reading; `JoyInput.read`
treats silence past JOY_TTL_S as absence, because that node dying looks like
silence rather than like an unplugged pad and the arm cannot tell them apart.

Both control sources are now normalised to a [-1, 1] six-vector before anything
is scaled, and the scaling happens in one function. The web path used to pass a
velocity straight through without going near mapping, which is why the console
had to speak the arm's units. It no longer does.

`twist` (SI) and `command` (robot-agnostic verbs) are the standard ports. `jog`
and `request` still carry the same intent in dora-ulite6's units and service
names so the two repositories can move independently; LEGACY_REQUEST is the
only place that knows what that node calls a verb. Wire one or the other, never
both. `joint_jog` is specified in the manifest and deliberately not built.

Schemas are declared rather than inferred: pyarrow reads a null `value` as its
own type, so an inferred `command` would change its port's type between one
verb and the next.

Three bugs found while moving this code, each with a test:

- `Sample.active` compared raw axes against a threshold, but SDL rests a
  trigger at -1.0, so every connected pad read as in use for ever: the pad took
  the lease on plug-in and GAMEPAD_IDLE_RELEASE_S never fired.
- The e-stop latch was never consulted when deciding a velocity, so Back
  stopped the arm for one tick and the next stick nudge drove it again. Nothing
  on the pad could clear it either. It is now checked above the lease, and X
  releases it and clears the controller's error, as the console's RESET does.
- Buttons were read before the arbiter granted the lease, so the first press
  after the pad had been idle was swallowed: not owned yet when read, and no
  longer a rising edge by the next tick.

tests/test_probe.py

@@ -1,7 +1,7 @@
 """The probe: what it draws, without a pad or a terminal."""
 
 from dora_teleop import mapping, probe
-from dora_teleop.gamepad import Sample
+from dora_teleop.joy import Sample
 from tests.conftest import stick
 
 
@@ -25,13 +25,13 @@ def test_a_reading_beyond_full_deflection_stays_on_the_bar():
     assert "#" in probe.bar(4.0)
 
 
-def test_no_gamepad_says_so_and_names_the_index_it_looked_for(config):
-    config.joystick_index = 2
-
+def test_no_gamepad_says_so_and_says_the_arm_is_being_held_still(config):
+    # Absence covers both "no pad" and "dora-joy stopped sending", and the probe
+    # must not imply the arm is waiting on something it is not.
     drawn = probe.render(Sample(present=False), [0.0] * 6, config)
 
-    assert "no gamepad found" in drawn
-    assert "index 2" in drawn
+    assert "no gamepad" in drawn
+    assert "hold still" in drawn
 
 
 def test_every_axis_the_pad_reports_is_shown_even_the_unused_ones(config):
@@ -79,3 +79,10 @@ def test_a_moving_arm_is_reported_as_moving(config):
     drawn = probe.render(stick(axes=[0.0, -1.0]), [10.0, 0, 0, 0, 0, 0], config)
 
     assert "moving: yes" in drawn
+
+
+def test_whether_the_pad_is_asking_for_the_arm_is_shown(config):
+    # The probe is where a resting trigger claiming the arm would have been
+    # visible, so it says what `is_active` thinks rather than leaving it implied.
+    assert "asking for the arm: no" in probe.render(stick(), [0.0] * 6, config)
+    assert "asking for the arm: yes" in probe.render(stick(axes=[0.0, -1.0]), [0.0] * 6, config)