Add single-shot measurement mode matching vendor protocol

Stop command triggers a one-shot reading instead of start+poll+stop.
spectrometer.py now uses take_single_reading() for this flow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hpcs6500.py

@@ -426,6 +426,30 @@ class HPCS6500:
 
         return result
 
+    def take_single_reading(self):
+        """Trigger a single measurement (no PSU control).
+        Sends stop to trigger a one-shot reading, waits for data, reads it,
+        then resets. This matches the vendor software's single-test flow."""
+        self.stop_measurement()
+
+        if not self.wait_for_data(timeout=30.0):
+            print("Timeout waiting for measurement data")
+            return None
+
+        meas = self.read_measurement_block()
+        elec = self.read_electrical_block()
+
+        if meas is None:
+            print("Failed to read measurement block")
+            return None
+
+        result = self._parse_measurement(meas)
+        if elec is not None:
+            result.update(self._parse_electrical(elec))
+
+        self.reset()
+        return result
+
     def read_current(self):
         """Read current data without starting/stopping measurement.
         Use this when the vendor software is controlling the instrument."""

spectrometer.py

@@ -17,7 +17,7 @@ def measure():
     try:
         dev.identify()
         dev.set_integration_time(0)  # auto
-        reading = dev.take_reading(psu=False)
+        reading = dev.take_single_reading()
     finally:
         dev.close()