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

include/app/xone_api.h

@@ -0,0 +1,24 @@
+#pragma once
+
+// ==============================================================================
+// C ABI bridge between the C++ protocol stack and the Swift app layer
+// ==============================================================================
+// This header is imported into Swift via -import-objc-header, so it must stay
+// pure C (no C++ types). The C++ modules implement these entry points.
+// ==============================================================================
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Human-readable version string of the protocol stack.
+const char *xone_version(void);
+
+// TRUE if an Xbox Wireless Dongle is connected, FALSE otherwise.
+bool xone_dongle_present(void);
+
+#ifdef __cplusplus
+}
+#endif