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>

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()