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

src/hid/hid_device.cpp

@@ -0,0 +1,7 @@
+// Virtual HID gamepad presentation
+// TODO(phase 4): DriverKit IOHIDDriver implementation.
+
+#include "hid/hid_device.hpp"
+
+namespace xone::hid {
+} // namespace xone::hid