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.

justfile

@@ -0,0 +1,19 @@
+set shell := ["bash", "-c"]
+
+default: run
+
+# Run the mux daemon
+run *args:
+    cargo run -- {{args}}
+
+# Run with verbose logging
+verbose *args:
+    cargo run -- --verbose {{args}}
+
+# Run clippy and tests
+check:
+    cargo clippy -- -D warnings && cargo test
+
+# Build release
+build:
+    cargo build --release