Use take_single_reading() for single measurements in CLI

Only use take_reading() (continuous-style) when --continuous flag is set.
Default single measurement now matches vendor single-test behavior.

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

hpcs6500.py

@@ -841,8 +841,10 @@ def main():
 
             if args.passive:
                 result = dev.read_current()
-            else:
+            elif args.continuous:
                 result = dev.take_reading(psu=False)
+            else:
+                result = dev.take_single_reading()
 
             if result is None:
                 print("Failed to get reading")