fix(chv-guest): 浏览器装系统路径（B-22）

Round 7 构建本身成功，但 BUILDINFO.versions.chromium 为空：
virt-customize 的客机 shell 里 HOME=/，Playwright 默认缓存
~/.cache/ms-playwright 落到根目录 /.cache（实测浏览器在
/.cache/ms-playwright/chromium_headless_shell-1248），而
build.sh 读的是 /root/.cache/ms-playwright（不存在），ls 读回
空文件；buildinfo.py 当时不校验 chromium 非空，静默放过，
事后 check_lockfile / generate --verify 兜底报未覆盖（exit 1）。
改：安装与读回都走 PLAYWRIGHT_BROWSERS_PATH=
/usr/local/share/ms-playwright（agent 运行时也须设同一变量，
否则 chv 用户在家目录找不到浏览器）；buildinfo.py 加
playwright_browsers_path 字段，并校验 playwright=1 时
versions.chromium 非空；README 同步。

chv-guest/buildinfo.py

@@ -35,6 +35,10 @@ BASE_CONTENTS = [
 # 否则换宿主/换执行器配置时无法复现。
 KERNEL_CMDLINE = "console=hvc0 root=/dev/vda1 rw"
 
+# 浏览器装在系统路径（B-22），不是某用户的 ~/.cache：构建时
+# PLAYWRIGHT_BROWSERS_PATH 指这里，agent 运行时也须设同一变量。
+PLAYWRIGHT_BROWSERS_PATH = "/usr/local/share/ms-playwright"
+
 
 def _read_lines(path):
     try:
@@ -92,6 +96,8 @@ def build(outdir, tag, ubuntu, codex_version, repo_commit,
         "ubuntu_image_sha256_set": True,
         "kernel_cmdline": KERNEL_CMDLINE,
         "playwright": bool(playwright),
+        "playwright_browsers_path": PLAYWRIGHT_BROWSERS_PATH
+        if playwright else "",
         "image_sha256": disk_sha,
         "artifacts": {
             "disk.raw": {"sha256": disk_sha, "size": disk_size},
@@ -124,6 +130,10 @@ def main(argv):
     if not info["gt_version"]:
         print("BUILDINFO gt_version 为空（gt 未装好）", file=sys.stderr)
         return 1
+    if playwright == "1" and not info["versions"].get("chromium"):
+        print("BUILDINFO versions.chromium 为空（浏览器未装好或"
+              "读回路径不对）", file=sys.stderr)
+        return 1
     if info["tag"] != tag:
         print("BUILDINFO tag 不一致", file=sys.stderr)
         return 1