refactor(detect): 可视化产出改为按类别分目录，与 generate_boxes 一致

原先按 split 和结果类型（ok/missed/extra/none）分目录，层级太深。改成
<类别>/<图名>.png，与 runs/boxes_shap 的结构一致。类别取真值里框最多的那个；
负样本没有真值框，按文件名前缀归类（<类别>_<序号> 是 prepare_label_studio 的命名），
认不出来放 background/。

split 与结果类型仍记在 index.csv 的列里，排序筛选即可。

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

tests/test_visualize_detections.py

@@ -87,3 +87,24 @@ def test_load_dataset_reports_a_missing_directory(dataset):
     dataset.write_text(yaml.safe_dump(meta), encoding="utf-8")
     with pytest.raises(FileNotFoundError, match="不存在"):
         vd.load_dataset(dataset)
+
+
+# ---------------- 产出的目录结构 ----------------
+
+
+def test_class_dir_uses_the_dominant_ground_truth_class():
+    classes = ["blackdot", "grease", "normal"]
+    truth = [from_yolo(1, .5, .5, .2, .2), from_yolo(0, .1, .1, .1, .1), from_yolo(1, .8, .8, .1, .1)]
+    assert vd.class_dir_for(truth, classes, "grease_0007") == "grease"
+
+
+def test_class_dir_falls_back_to_the_filename_prefix():
+    """负样本没有真值框，按 <类别>_<序号> 的命名归类。"""
+    classes = ["blackdot", "grease", "normal"]
+    assert vd.class_dir_for([], classes, "normal_1234") == "normal"
+
+
+def test_class_dir_falls_back_to_background_for_unknown_names():
+    classes = ["blackdot", "grease", "normal"]
+    assert vd.class_dir_for([], classes, "IMG_20260101") == "background"
+    assert vd.class_dir_for([], classes, "nofix") == "background"