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/discovery.rs

@@ -0,0 +1,47 @@
+//! USB device discovery for DongLoRa dongles.
+//!
+//! Finds the serial port by matching USB VID:PID using the `serialport` crate.
+
+use std::thread;
+use std::time::Duration;
+
+use tracing::info;
+
+/// DongLoRa USB Vendor ID.
+pub const USB_VID: u16 = 0x1209;
+
+/// DongLoRa USB Product ID.
+pub const USB_PID: u16 = 0x5741;
+
+/// Find the DongLoRa serial port by USB VID:PID.
+///
+/// Returns the first matching port path, or `None` if no device is found.
+pub fn find_port() -> Option<String> {
+    let ports = serialport::available_ports().ok()?;
+    ports
+        .into_iter()
+        .find(|p| {
+            matches!(
+                &p.port_type,
+                serialport::SerialPortType::UsbPort(info)
+                    if info.vid == USB_VID && info.pid == USB_PID
+            )
+        })
+        .map(|p| p.port_name)
+}
+
+/// Block until a DongLoRa device appears on USB.
+///
+/// Polls [`find_port`] every 500ms and returns the port path once found.
+pub fn wait_for_device() -> String {
+    info!("waiting for DongLoRa device...");
+    loop {
+        if let Some(port) = find_port() {
+            info!("found device at {port}");
+            // Brief delay for USB enumeration to settle
+            thread::sleep(Duration::from_millis(300));
+            return port;
+        }
+        thread::sleep(Duration::from_millis(500));
+    }
+}