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/gip/protocol.hpp

@@ -0,0 +1,15 @@
+#pragma once
+
+// ==============================================================================
+// GIP protocol (Game Input Protocol)
+// ==============================================================================
+// Port target: medusalix/xone bus/protocol.c + bus/bus.c.
+// Client lifecycle, ANNOUNCE/IDENTIFY handshake, auth handshake, status
+// reports (battery, connected), HID report pass-through.
+// ==============================================================================
+
+namespace xone::gip {
+
+// TODO(phase 1): GIP frame types, client lifecycle, handshake state machine.
+
+} // namespace xone::gip