Macro host 

macro_rules! host {
    () => { ... };
}

Pull the build-time registration plan into the crate root. 把构建期捕获的注册计划引入 crate 根。

Call it once per host crate whose build script has run the NichLink build step; the included file supplies builtin_static_plan() and the per-level object aliases. The include! target lives under OUT_DIR, so a crate that never runs that step fails to compile rather than silently missing faces. 在已由 build script 运行 NichLink 构建步骤的宿主 crate 中调用一次；被包含的文件 提供 builtin_static_plan() 与各层级 object 别名。include! 目标位于 OUT_DIR， 未运行该步骤的 crate 会编译失败，而不是静默缺少注册面。