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

.gitignore

@@ -0,0 +1,40 @@
+# CMake
+build
+build-*
+cmake-build-*
+__cmake*
+
+# Firmware binaries (downloaded via scripts/download-firmware.sh)
+firmware/*.bin
+
+# Xcode
+*.xcworkspace
+*.xcodeproj
+
+# Visual Studio
+*.sln
+*.vcxproj
+*.vcxproj.filters
+*.vcxproj.user
+.vs
+
+# Makefile
+Makefile
+*.make
+
+# nvim
+.ccls
+.ccls-cache
+.cache
+compile_commands.json
+
+# Visual Studio Code
+.vscode
+
+# IntelliJ
+.idea
+
+# macOS
+.DS_Store
+.AppleDouble
+.LSOverride