Helios diode current: stop the status poll overwriting the setpoint

The current spin box could not hold a typed value: the 1 Hz status poll
read LDS and wrote it straight into the spin box, so the operator's number
was replaced by the laser's within a second — before Set could be pressed.
The spin box is now seeded once on connect and belongs to the operator
after that; the poll's reading goes to a read-back label beside the Set
button, so the value about to be sent and the value the laser holds are
separate readouts.

The write itself was also unverified. Section 6 of the operator's manual:
"Commands or set values can be discarded by the controller unintentionally.
It is recommended to query the set value after the command is entered to
confirm the actual value." set_current_ma() wrote LDS and returned True
regardless, so a discarded write looked exactly like a good one.
_write_verified() now writes, reads back, and retries up to three times;
set_current_ma() and set_frequency_hz() use it, and HeliosWorker reports a
refusal on the status line instead of echoing the requested value.

Also from the manual, recorded but not acted on: LDS accepts 0-7000 mA
(the driver's 2000 mA ceiling is this rig's, not the protocol's), LDF has
to be re-sent after LDG changes, and the power-monitor mnemonic is HMP —
which this laser does not implement, per the operator.

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

sc3-aui-helios.ui

@@ -109,6 +109,29 @@
         </property>
        </widget>
       </item>
+      <item>
+       <widget class="QLabel" name="helios_current_readback_label">
+        <property name="toolTip">
+         <string>The diode current the controller reports. The spin box is the value that will be sent when Set is pressed.</string>
+        </property>
+        <property name="text">
+         <string>laser: --- mA</string>
+        </property>
+       </widget>
+      </item>
+      <item>
+       <spacer name="horizontalSpacer_current">
+        <property name="orientation">
+         <enum>Qt::Orientation::Horizontal</enum>
+        </property>
+        <property name="sizeHint" stdset="0">
+         <size>
+          <width>40</width>
+          <height>20</height>
+         </size>
+        </property>
+       </spacer>
+      </item>
      </layout>
     </widget>
    </item>