fix: connect layer change to update field combo boxes (#111)

* test: pin training-value/isovalue direction agreement for stratigraphic columns

Guards against the swap fixed in 814be12: model_manager.py's per-unit
training value and LoopStructural's get_isovalues() must agree on
which direction values increase, or extracted isosurfaces get labelled
with the wrong unit while keeping correct geometry.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix: connect layer change to update field combo boxes

* style: fix black formatting and ruff lint (unnecessary generator)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: lachlangrose

loopstructural/gui/map2loop_tools/sorter_widget.py

@@ -70,6 +70,7 @@ def __init__(self, parent=None, data_manager=None, debug_manager=None):
         self.sortingAlgorithmComboBox.currentIndexChanged.connect(self._on_algorithm_changed)
         self.geologyLayerComboBox.layerChanged.connect(self._on_geology_layer_changed)
         self.structureLayerComboBox.layerChanged.connect(self._on_structure_layer_changed)
+        self.contactsLayerComboBox.layerChanged.connect(self._on_contacts_layer_changed)
         self.runButton.clicked.connect(self._run_sorter)
         self.orientationTypeComboBox.setCurrentIndex(1)  # Default to Dip Direction
         self._guess_layers()
@@ -278,6 +279,12 @@ def _on_structure_layer_changed(self):
             if dipdir_match := matcher.find_match('DIPDIR'):
                 self.dipDirFieldComboBox.setField(dipdir_match)
 
+    def _on_contacts_layer_changed(self):
+        """Update field combo boxes when contacts layer changes."""
+        layer = self.contactsLayerComboBox.currentLayer()
+        self.unitName1FieldComboBox.setLayer(layer)
+        self.unitName2FieldComboBox.setLayer(layer)
+
     def _on_algorithm_changed(self):
         """Update UI based on selected sorting algorithm and map2loop requirements."""
         algorithm_index = self.sortingAlgorithmComboBox.currentIndex()