Function validate_graft_selector 

pub fn validate_graft_selector(
    selector: &str,
) -> Result<(), GraftPlanDocumentError>

Check one external implementation selector against the plan format. 按计划格式校验一个外部实现选择器。

One rule, shared by the file writer and the parser, so a plan this crate writes is always a plan this crate can read back. 写入方与解析方共用同一条规则，因此本 crate 写出的计划永远读得回来。