refactor: reorganize backend modules into sub-packages for models, data, visualization, wafer, and controllers

tests/test_threshold_classifier.py

@@ -1,6 +1,6 @@
 import math
 
-from pygui.backend.threshold_classifier import (
+from pygui.backend.models.threshold_classifier import (
     ThresholdConfig, classify, classify_all, resolve_bounds,
     BAND_IN, BAND_HIGH, BAND_LOW
 )