Add rust-bundle, a single staticlib crate that depends on both libchat
and rln as rlibs. This ensures rustc links Rust std exactly once,
eliminating duplicate symbol errors on all platforms. Nim targets link
against liblogos_chat_bundle.a; Nix uses a bundleDrv instead of
separate libchat and rln derivations.
Reference: https://doc.rust-lang.org/reference/linkage.html#mixed-rust-and-foreign-codebases
