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/mt76/mt76.cpp

@@ -0,0 +1,16 @@
+// MT76 chip protocol (MediaTek MT76xx radio in the dongle)
+// TODO(phase 3): port from medusalix/xone transport/mt76.c.
+
+#include "mt76/mt76.hpp"
+
+namespace xone::mt76 {
+
+auto load_firmware(std::uint16_t pid, char const *firmware_path) -> bool
+{
+    (void)pid;
+    (void)firmware_path;
+    // Not implemented yet: no USB transport in place.
+    return false;
+}
+
+} // namespace xone::mt76