docs: add README and LICENSE to Rust client and mux crates

LICENSE

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+MIT License
+
+Copyright (c) 2025 Stephen Waits <steve@waits.net>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# DongLoRa Mux (Rust)
+
+USB multiplexer daemon — lets multiple applications share one DongLoRa
+dongle simultaneously.
+
+## What It Does
+
+- Owns the USB serial connection exclusively
+- Exposes a Unix domain socket (and optional TCP) speaking the same
+  COBS-framed protocol
+- RxPacket frames broadcast to all connected clients
+- StartRx/StopRx reference-counted across clients
+- SetConfig locked once set (single client can change freely)
+- Automatic dongle reconnect on hot-plug
+- No panics — enforced by clippy deny lints
+
+## Running
+
+```sh
+just run                                    # start the mux daemon
+just verbose                                # start with verbose logging
+just run -- --tcp 5741 --port /dev/ttyACM0  # with options
+```
+
+## Depends On
+
+- [clients/rust](../../clients/rust/) — the `donglora-client` library
+  (protocol types, COBS framing, device discovery)