fix(backbones): 修复 shufflenet 无法构建，并让换骨架不再踩配置合并的坑

- shufflenet_v2_x1_0 列在支持清单里却直接 AttributeError：ShuffleNetV2 既不是
  扁平 children 也没有 .features，需要按 conv1/maxpool/stage2..4/conv5 自己拼。
  该 bug 同时影响 SimCLR 与检测模型
- 新增参数化测试覆盖清单里的每个 arch：能前向、声明的 out_channels 与实际一致
- 配置支持 `_replace_: true`：子字典整块替换而不与 base 合并。换骨架时
  tiny_conv 的 widths/depths 会残留并传给新骨架，此前无论用 --set 还是 _base_
  都绕不开，只能改原文件
- Registry.build 提前校验字段，报错直接指出是哪些字段不被接受、该怎么办，
  取代裸 TypeError
- README 补充可选骨架表与三种改法（含不该用 --set 改 name 的说明）

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

tests/test_config.py

@@ -46,3 +46,40 @@ def test_project_configs_are_loadable():
     assert cfg["model"]["name"] == "defect_net"
     assert cfg["data"]["train"]["dataset"]["name"] == "film_defect"
     assert len(cfg["classes"]) > 0
+
+
+def test_replace_key_swaps_a_whole_subdict(tmp_path):
+    """换组件时要能整块替换，否则旧组件的专有字段会残留下来传给新组件。"""
+    (tmp_path / "base.yaml").write_text(
+        "backbone:\n  name: tiny_conv\n  widths: [32, 64]\n  depths: [1, 2]\n  block: dw\n",
+        encoding="utf-8",
+    )
+    (tmp_path / "child.yaml").write_text(
+        "_base_: ./base.yaml\nbackbone:\n  _replace_: true\n  name: torchvision\n  arch: resnet18\n",
+        encoding="utf-8",
+    )
+
+    cfg = load_config(tmp_path / "child.yaml")
+
+    assert cfg["backbone"] == {"name": "torchvision", "arch": "resnet18"}
+    assert "_replace_" not in cfg["backbone"]  # 标记本身不能漏进构造参数
+
+
+def test_without_replace_key_subdicts_still_merge(tmp_path):
+    (tmp_path / "base.yaml").write_text("backbone:\n  name: tiny_conv\n  block: dw\n", encoding="utf-8")
+    (tmp_path / "child.yaml").write_text(
+        "_base_: ./base.yaml\nbackbone:\n  block: res\n", encoding="utf-8"
+    )
+
+    cfg = load_config(tmp_path / "child.yaml")
+    assert cfg["backbone"] == {"name": "tiny_conv", "block": "res"}
+
+
+def test_registry_reports_incompatible_fields_actionably():
+    """--set 只改 name 时旧字段会残留，报错要说清楚是哪些字段、该怎么办。"""
+    import pytest
+
+    from models.registry import BACKBONES
+
+    with pytest.raises(TypeError, match=r"不接受的字段.*widths"):
+        BACKBONES.build({"name": "torchvision", "arch": "resnet18", "widths": [32]})