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

deps/Platform.cmake

@@ -0,0 +1,54 @@
+# ==============================================================================
+# Platform Detection
+# ==============================================================================
+# This module detects the current platform and compiler, setting IS_* variables
+# that can be used throughout the build system for conditional logic.
+#
+# Compiler flags set:
+#   IS_CLANG_OR_GCC  - TRUE if using Clang or GCC compiler
+#   IS_MSVC          - TRUE if using Microsoft Visual C++ compiler
+#
+# Platform flags set:
+#   IS_WINDOWS       - TRUE if building for Windows
+#   IS_LINUX         - TRUE if building for Linux
+#   IS_MACOS         - TRUE if building for macOS
+#   IS_IOS           - TRUE if building for iOS
+# ==============================================================================
+
+# ------------------------------------------------------------------------------
+# Compiler Detection
+# ------------------------------------------------------------------------------
+set(IS_CLANG_OR_GCC FALSE)
+set(IS_MSVC         FALSE)
+
+if(MSVC)
+    set(IS_MSVC TRUE)
+elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
+    set(IS_CLANG_OR_GCC TRUE)
+endif()
+
+# ------------------------------------------------------------------------------
+# Platform Detection
+# ------------------------------------------------------------------------------
+set(IS_WINDOWS FALSE)
+set(IS_LINUX   FALSE)
+set(IS_MACOS   FALSE)
+set(IS_IOS     FALSE)
+
+if(ANDROID)
+    message(FATAL_ERROR "Android is not supported by xone_macos!")
+elseif(APPLE)
+    if(IOS)
+        message(STATUS "Platform: iOS")
+        set(IS_IOS TRUE)
+    else()
+        message(STATUS "Platform: macOS")
+        set(IS_MACOS TRUE)
+    endif()
+elseif(WIN32)
+    message(FATAL_ERROR "xone_macos only builds on macOS!")
+elseif(UNIX)
+    message(FATAL_ERROR "xone_macos only builds on macOS!")
+else()
+    message(FATAL_ERROR "Unknown platform!")
+endif()