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_app.py

@@ -399,8 +399,19 @@ class HeliosWorker(PollingQueueWorker):
     def _do_set_current(self, current_ma: int):
         if not self._laser:
             return
-        self._laser.set_current_ma(current_ma)
-        self.current_updated.emit(current_ma)
+        if self._laser.set_current_ma(current_ma):
+            self.current_updated.emit(current_ma)
+            return
+        # The manual warns the controller drops set values unintentionally,
+        # so the driver verifies the write.  Say when it did not take: this
+        # used to emit the requested value regardless, and the next poll
+        # then quietly restored the old one.
+        actual = self._laser.get_current_ma()
+        if actual is not None:
+            self.current_updated.emit(actual)
+        self.error_occurred.emit(
+            f"Diode current: controller kept {actual} mA, "
+            f"did not accept {current_ma} mA")
 
     def _poll_once(self):
         """Read the full status set.  Individual reads are allowed to fail
@@ -982,6 +993,7 @@ class HeliosWindow(QWidget):
         self._worker = worker
         # Polling is driven by the worker itself (self-rescheduling), so
         # there is no timer here to outpace the device.
+        self._seed_current_spin = False
         self._wire_signals()
         self._update_controls(False)
         self._set_emission_indicator(False)
@@ -1029,6 +1041,8 @@ class HeliosWindow(QWidget):
             self._worker.queue_disconnect()
 
     def _on_connected(self):
+        # Take the setpoint from the laser once, on connect.
+        self._seed_current_spin = True
         self.helios_connect_toggle.setEnabled(True)
         self.helios_connect_toggle.setText("Disconnect")
         self.helios_status_label.setText("Connected")
@@ -1077,9 +1091,20 @@ class HeliosWindow(QWidget):
         self._worker.queue_set_current(self.helios_current_spin.value())
 
     def _on_current_updated(self, current_ma: int):
-        self.helios_current_spin.blockSignals(True)
-        self.helios_current_spin.setValue(current_ma)
-        self.helios_current_spin.blockSignals(False)
+        """Show what the laser reports; leave the spin box to the operator.
+
+        The 1 Hz poll used to write its reading straight into the spin box,
+        so a number typed there was overwritten within a second — the
+        setpoint appeared to snap back to the controller's value before it
+        could be sent.  The spin box is now seeded once per connection and
+        is the operator's alone after that; the label is the read-back.
+        """
+        self.helios_current_readback_label.setText(f"laser: {current_ma} mA")
+        if self._seed_current_spin:
+            self._seed_current_spin = False
+            self.helios_current_spin.blockSignals(True)
+            self.helios_current_spin.setValue(current_ma)
+            self.helios_current_spin.blockSignals(False)
 
     def _reset_temperatures(self):
         self.helios_temp_diode_label.setText("--- °C")
@@ -1101,6 +1126,8 @@ class HeliosWindow(QWidget):
             self.helios_enable_btn.setText("Enable Laser")
             self.helios_enable_btn.blockSignals(False)
             self._set_emission_indicator(False)
+            self._seed_current_spin = False
+            self.helios_current_readback_label.setText("laser: --- mA")
             self._reset_temperatures()
             self.helios_ler_label.setText("---")
             self.helios_lce_label.setText("---")