finish refactor, add docs and CI

README.md

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+# NXST
+
+Transfer save data between two Nintendo Switch consoles over local Wi-Fi.
+
+Pick a game, pick a user. One Switch sends; the other receives. The sender backs up its own save first, so you can never lose data in transit.
+
+---
+
+## Install
+
+1. Download `NXST.nro` from [Releases](../../releases).
+2. Copy it to `/switch/NXST/NXST.nro` on your SD card.
+3. Launch via hbmenu (hold R while starting any game, or Album).
+
+Both Switches must be on the same Wi-Fi network. No router configuration needed — discovery uses UDP multicast.
+
+---
+
+## Usage
+
+**Sender** (the Switch whose save you want to copy):
+
+1. Open NXST → select a title → press **A** → **Transfer**.
+2. Wait for "Waiting for receiver…" to change to "Transferring…".
+
+**Receiver** (the Switch that will receive the save):
+
+1. Open NXST → select the same title → press **A** → **Receive**.
+2. Wait. The save is restored automatically when the transfer finishes.
+
+Press **B** on either side to cancel mid-transfer.
+
+Logs are written to `/switch/NXST/log.log`.
+
+---
+
+## Build
+
+**Prerequisites:** [devkitPro](https://devkitpro.org/wiki/Getting_Started) with `switch-dev` and `switch-portlibs` packages, plus `cmake ≥ 3.20`.
+
+```bash
+# Clone with submodules (Plutonium UI)
+git clone --recurse-submodules https://github.com/your-username/NXST.git
+cd NXST
+
+# Configure (once)
+cmake --preset switch
+
+# Build
+cmake --build build
+
+# Send to Switch via nxlink (Switch must be on same network, nxlink running)
+cmake --build build --target send
+```
+
+Output: `build/NXST.nro`
+
+---
+
+## Architecture
+
+See [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) for the layer diagram, threading model, and key types.
+
+See [`docs/PROTOCOL.md`](docs/PROTOCOL.md) for the wire protocol (UDP multicast discovery + TCP file stream).
+
+```
+ui/          — TitlesLayout, UsersLayout, TransferOverlay, HeaderBar
+service/     — TransferService (all network threads and state)
+infra/net/   — Socket RAII, sendAll/recvAll
+infra/fs/    — io::backup, io::restore, directory iterator, RAII handles
+infra/sys/   — nxst::log (printf-checked, timestamped)
+domain/      — Title, Account, Result<T>, TransferState, protocol constants
+```
+
+---
+
+## Credits
+
+- **[Plutonium](https://github.com/XorTroll/Plutonium)** — Switch UI framework by XorTroll
+- **[Checkpoint](https://github.com/BernardoGiordano/Checkpoint)** — save management library by Bernardo Giordano / FlagBrew; several files in `src/infra/fs/` and `src/domain/` are derived from Checkpoint
+
+---
+
+## License
+
+GPLv3 — see [`LICENSE`](LICENSE).
+
+NXST includes code derived from Checkpoint (GPLv3). All original NXST code is released under the same license to satisfy the GPL inheritance requirement.