feat: add Rust client library and mux daemon

Add donglora-client (clients/rust/) and donglora-mux (mux/rust/) as
Rust implementations of the existing Python client library and mux
daemon. Both are standalone Cargo projects with no-panic guarantees
enforced by clippy deny lints.

donglora-client provides protocol types, COBS framing, USB device
discovery, serial/mux transports, and a high-level Client<T> with
send/recv. donglora-mux is a fully async (tokio) multiplexer daemon
with dongle hot-plug reconnect, per-client bounded send queues,
SetConfig locking, and StartRx/StopRx reference counting.

52 unit tests across both crates. Updates README roadmap checkboxes
and CONTRIBUTING docs to reflect the new implementations.

CONTRIBUTING.md

@@ -22,8 +22,8 @@ common tasks consume. No shared board code — each board is self-contained.
 
 ## Client Libraries
 
-The Python library (`clients/python/`)
-is the reference implementation. To add a library in another language:
+Client libraries exist in [Python](clients/python/) and [Rust](clients/rust/).
+To add a library in another language:
 
 1. Create a directory with the language's standard project layout
 2. Implement the wire protocol from [firmware/PROTOCOL.md](firmware/PROTOCOL.md)