Fix underlying2scaled formula: (data - offset) * scale

MATLAB's underlying2scaled does (d - offset) * scale, not d * scale + offset.
With offset=32768 and scale=0.195, this converts raw Intan ADC values
to microvolts correctly.

https://claude.ai/code/session_01A7rAxYf5pSvs19iVJe3ncL

Author: claude

src/ndi/daq/mfdaq.py

@@ -766,8 +766,8 @@ def readchannels_epochsamples_ingested(
         if count < data.shape[0]:
             data = data[:count, :]
 
-        # Apply offset and scale
-        data = data * np.array(scale) + np.array(offset)
+        # Apply offset and scale: underlying2scaled formula is (data - offset) * scale
+        data = (data - np.array(offset)) * np.array(scale)
 
         return data