Allow blog posts to specify an explicit URL slug

I have some posts with non-English titles, but I'd like their URLs to use short ASCII slugs. Currently the sulg is computed from the post title:

https://github.com/leanprover/verso/blob/ffd58ff11562c7af8698dac150f3a04d559ab201/src/verso-blog/VersoBlog/Basic.lean#L222-L231

While overriding postName can customize the generated name, it only gets the date and title, so the slug has to live somewhere else. And more importantly, Config now seems to be an internal thing:

https://github.com/leanprover/verso/blob/ffd58ff11562c7af8698dac150f3a04d559ab201/src/verso-blog/VersoBlog.lean#L843-L849

There's no way to pass it as argument, so I'm not sure how to modify postName.

I thought it would be nice to add an optional slug to Post.Meta, so this would get /blog/1980-01-01-test/ naturally:

#doc (Post) "测试" =>

%%%
authors := ["foo"]
date := { year := 1980, month := 1, day := 1 }
slug := "test"
%%%

But I'm not sure if there's already a good way to extend post metadata downstream (without defining a new blog genre).

Read the slug generation in src/verso-blog/VersoBlog/Basic.lean#L222-L231 and the Config definition in src/verso-blog/VersoBlog.lean#L843-L849. Trace how Post.Meta is parsed and how postName receives metadata, then determine how an optional slug can produce the requested ASCII URL while remaining usable by downstream blog authors.