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.

src/lib.rs

@@ -0,0 +1,45 @@
+//! DongLoRa host library — connect, configure, send/receive LoRa packets.
+//!
+//! Implements the DongLoRa USB protocol (COBS-framed fixed-size LE).
+//! See `firmware/PROTOCOL.md` for the full specification.
+//!
+//! # Quick start
+//!
+//! ```no_run
+//! use donglora_client::*;
+//!
+//! let mut client = connect_default()?;
+//! client.ping()?;
+//! client.set_config(RadioConfig::default())?;
+//! client.start_rx()?;
+//!
+//! loop {
+//!     if let Some(Response::RxPacket { rssi, snr, payload }) = client.recv()? {
+//!         println!("RX rssi={rssi} snr={snr} len={}", payload.len());
+//!     }
+//! }
+//! # Ok::<(), anyhow::Error>(())
+//! ```
+
+pub mod client;
+pub mod codec;
+pub mod connect;
+pub mod discovery;
+pub mod protocol;
+pub mod transport;
+
+// Flat re-exports for convenience
+pub use client::Client;
+pub use codec::{decode_frame, encode_frame, read_frame, FrameReader};
+pub use connect::{connect, connect_default, default_socket_path};
+pub use discovery::{find_port, wait_for_device, USB_PID, USB_VID};
+pub use protocol::{
+    Bandwidth, Command, ErrorCode, RadioConfig, Response, CMD_TAG_SET_CONFIG, CMD_TAG_START_RX,
+    CMD_TAG_STOP_RX, ERROR_INVALID_CONFIG, MAX_PAYLOAD, PREAMBLE_DEFAULT, RADIO_CONFIG_SIZE,
+    RESP_TAG_ERROR, RESP_TAG_OK, RESP_TAG_RX_PACKET, TX_POWER_MAX,
+};
+pub use transport::{AnyTransport, MuxTransport, SerialTransport, Transport};
+
+#[cfg(unix)]
+pub use connect::mux_connect;
+pub use connect::mux_tcp_connect;