fix(chv-muse-r12fix-4): detect_checkout 接进 CDP broker（P10）

R12 真机：detect_checkout 在 chv-guest/、host.py、
executor.py 里零引用，结账页无任何拦截。按 P10 gap §4.1
接线：broker 加载 cloud/chv/checkout_detect.py（同目录
优先，仓库路径兜底；缺失时降级关闭检测、写警告，绝不
crash-loop）；snapshot() 每次重算 verdict（aria 行拼伪节点，
与 _form_risk 同形），open() 导航前后两道门——落点是结账
页时返回 form_submit ask 并暂停，上一页是结账页时下一次
导航先审批；ask 批过后同 URL 不再重复 ask（批准即放行，
与 form_risk 同口径）；verdict 进 snapshot()/status() 回包。
build.sh：checkout_detect.py 条件拷入 /usr/local/bin/（0644，
P3/P6 同款只警告口径，M4 不硬要求 cloud/），
extra_contents 与两处 lockfile 登记补齐。钱包三腿不在本轮
范围，未动。

cloud/tests/test_chv_muse_p10_checkout.py

@@ -6,7 +6,10 @@
 - 中英文特征；节点为空；页面金额只作展示信号。
 """
 
+import importlib.util
+import os
 import sys
+import tempfile
 import unittest
 from pathlib import Path
 
@@ -129,5 +132,43 @@ class CheckoutNodesTest(unittest.TestCase):
         self.assertIsNone(verdict.amount_text)
 
 
+class BuildInfoContentsTest(unittest.TestCase):
+    """round-12 修复轮（项 4，P10）：checkout_detect 是条件拷入
+    件——存在时经 extra_contents 登记进 BUILDINFO contents
+    （不在 BASE_CONTENTS 里，缺失的构建绝不谎称已进镜像，
+    与 P3 叶子模块同款口径）。"""
+
+    def test_extra_contents_include_checkout_detect(self):
+        root = Path(__file__).resolve().parents[2]
+        path = root / "chv-guest" / "buildinfo.py"
+        spec = importlib.util.spec_from_file_location(
+            "chv_guest_p10_buildinfo", str(path))
+        module = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(module)
+        self.assertNotIn("checkout_detect",
+                         module.BASE_CONTENTS)
+        with tempfile.TemporaryDirectory() as tmp:
+            out = os.path.join(tmp, "out")
+            os.makedirs(out)
+            for name, content in (("disk.raw", b"D"),
+                                  ("vmlinux", b"K")):
+                with open(os.path.join(out, name), "wb") as fh:
+                    fh.write(content)
+            for name, lines in (
+                    ("guest-versions-dpkg.txt", ["git 9.9.9-9ubuntu9"]),
+                    ("guest-versions-pip.txt", ["playwright==9.9.9"]),
+                    ("guest-versions-gt.txt", ["gt version 1.0.0"]),
+                    ("guest-versions-chromium.txt",
+                     ["chromium_headless_shell-1248"])):
+                with open(os.path.join(out, name), "w") as fh:
+                    fh.write("\n".join(lines) + "\n")
+            info = module.build(
+                out, "chv-guest-24.04-0.164.0-alpha.9-abc1234", "24.04",
+                "0.164.0-alpha.9", "abc1234", "0" * 64, True, "1248",
+                ["cloud/chv/checkout_detect.py"])
+            self.assertIn("cloud/chv/checkout_detect.py",
+                          info["contents"])
+
+
 if __name__ == "__main__":
     unittest.main()