fix: stabilize cold display occupancy detection

tests/test_vision.py

@@ -10,6 +10,7 @@ from cold_display_guard.vision import (
     RuntimeVisionSettings,
     ZoneOccupancyDetector,
     load_runtime_vision_settings,
+    metrics_indicate_occupied,
     point_in_polygon,
 )
 
@@ -157,6 +158,7 @@ class VisionTests(unittest.TestCase):
 
         self.assertEqual(settings.sample_stride_pixels, 4)
         self.assertEqual(settings.occupancy_mean_delta, 55.0)
+        self.assertEqual(settings.occupancy_absolute_dark_fraction, 0.0)
         self.assertEqual(settings.occupancy_confirm_frames, 2)
         self.assertEqual(settings.empty_confirm_frames, 2)
         self.assertEqual(settings.trash_motion_delta, 18.0)
@@ -164,6 +166,25 @@ class VisionTests(unittest.TestCase):
         self.assertEqual(settings.trash_sustained_motion_frames, 2)
         self.assertEqual(settings.trash_motion_cooldown_seconds, 3)
 
+    def test_absolute_dark_fraction_can_detect_food_already_present_in_baseline(self) -> None:
+        settings = RuntimeVisionSettings(
+            occupancy_mean_delta=55,
+            occupancy_texture_delta=18,
+            occupancy_dark_fraction=0.06,
+            occupancy_absolute_dark_fraction=0.085,
+        )
+
+        occupied = metrics_indicate_occupied(
+            settings,
+            mean_delta=5.0,
+            texture_delta=0.5,
+            dark_fraction=0.09,
+            baseline_dark_fraction=0.10,
+            bright_fraction=0.0,
+        )
+
+        self.assertTrue(occupied)
+
     def test_detector_can_seed_previous_baseline_and_occupancy(self) -> None:
         detector = ZoneOccupancyDetector(
             [Region("1", ((0.0, 0.0), (1.0, 0.0), (1.0, 1.0), (0.0, 1.0)))],