jj-vcs/jj-vfs-poc ? reverse-engineered prompt

Build me a proof of concept virtual file system for jj in Rust.

I want it to act like a working reference implementation that jj can use to present repository contents through a virtual filesystem instead of normal checked out files. It doesn’t need to be perfect or production ready, but it should be clear, usable, and easy to understand. Please include a small set of tests so I can see the basic behavior, and keep the code organized well enough that it could serve as an intern project example.

If you need to check the latest jj or Rust docs while wiring it up, go ahead and do that. Also make sure every source file has the proper copyright and Apache license header.