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/app/main.swift

@@ -0,0 +1,13 @@
+// xone_macos — macOS app entry point (Swift)
+// TODO(phase 5): device discovery, pairing UI, status display, menubar icon.
+
+import Foundation
+
+let version = String(cString: xone_version())
+print("xone_macos \(version)")
+
+if xone_dongle_present() {
+    print("Xbox Wireless Dongle detected")
+} else {
+    print("No Xbox Wireless Dongle detected (plug in the dongle and retry)")
+}