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/usb/usb_transport.cpp

@@ -0,0 +1,14 @@
+// USB transport (IOKit / IOUSBFamily)
+// TODO(phase 2): port from medusalix/xone transport/dongle.c + mt76.c USB calls.
+
+#include "usb/usb_transport.hpp"
+
+namespace xone::usb {
+
+auto probe() -> bool
+{
+    // Not implemented yet: no IOKit device matching in place.
+    return false;
+}
+
+} // namespace xone::usb