chore: scaffold CMake build system

Add the CMake build for the Swift + C++ macOS port, mirroring the
refix layout: deps/ modules (Platform, Flags, Sanitizers, FindUnity),
per-module static libraries (usb, auth, mt76, gip, hid, api), a Swift
app entry point with a pure C bridge header, and a Unity test suite
behind BUILD_TESTING.

Swift requires the Ninja or Xcode generator; a guard in
CMakeLists.txt rejects anything else.

Co-Authored-By: qwen (qwen/qwen3.8-27b@q2_k_xl): scaffolded CMake build + tests

README.md

@@ -7,6 +7,28 @@ See [PLAN.md](PLAN.md) for full architecture and implementation phases.
 
 ---
 
+## Building
+
+Requires CMake ≥ 3.21, Xcode (or Command Line Tools), and Ninja.
+Swift is only supported by the **Ninja** and **Xcode** generators in CMake,
+so configure with one of them explicitly:
+
+```sh
+cmake -S . -B build -G Ninja -DBUILD_TESTING=ON
+cmake --build build
+./build/xone_app
+cmake --build build --target check   # run the test suite
+```
+
+Layout (mirrors `../refix/`):
+
+- `deps/` — CMake modules: `Platform.cmake`, `Flags.cmake`, `Sanitizers.cmake`, `FindUnity.cmake`
+- `src/{usb,mt76,gip,auth,hid}` — C++ protocol stack (ported from `medusalix/xone`)
+- `src/app/` — Swift app entry point + C ABI bridge (`include/app/xone_api.h`)
+- `tests/` — Unity test suite (`BUILD_TESTING=ON`)
+
+---
+
 ## Investigation Checklist
 
 These items need research on macOS before implementation can begin.