fix(demo): harden mask hole-filling for border-touching cases

tests/demo/test_preprocess.py

@@ -95,6 +95,34 @@ class TestMaskToSilhouette:
         np.testing.assert_array_equal(result1, result2)
         np.testing.assert_array_equal(result2, result3)
 
+    def test_hole_inside_mask_is_filled(self) -> None:
+        h, w = 200, 160
+        mask = np.zeros((h, w), dtype=np.uint8)
+        mask[30:170, 40:120] = 255
+        mask[80:120, 70:90] = 0
+        bbox = (40, 30, 120, 170)
+
+        result = mask_to_silhouette(mask, bbox)
+
+        assert result is not None
+        result_arr = cast(NDArray[np.float32], result)
+        hole_patch = result_arr[26:38, 18:26]
+        assert float(np.mean(hole_patch)) > 0.8
+
+    def test_hole_fill_works_when_mask_touches_corner(self) -> None:
+        h, w = 220, 180
+        mask = np.zeros((h, w), dtype=np.uint8)
+        mask[0:180, 0:130] = 255
+        mask[70:120, 55:95] = 0
+        bbox = (0, 0, 130, 180)
+
+        result = mask_to_silhouette(mask, bbox)
+
+        assert result is not None
+        result_arr = cast(NDArray[np.float32], result)
+        hole_patch = result_arr[24:40, 16:28]
+        assert float(np.mean(hole_patch)) > 0.75
+
     def test_tall_narrow_mask_valid_output(self) -> None:
         """Tall narrow mask should produce valid silhouette."""
         h, w = 400, 50