Server functions in Fitz: call the server from a WASM component, no plumbing

TL;DR — Mark an async fn with @rpc and call it from a .fitzv component as if it were local: let u = get_user(42).await?. The compiler generates both halves from that one declaration — a POST /__rpc/get_user endpoint on the server (which can touch the DB, auth, secrets) and a fetch-based stub on the client — with the same type shared back and...