Compiling llama.cpp into a universal XCFramework for macOS and iOS

Got llama.cpp compiling into a universal XCFramework that supports both macOS and iOS — and the journey there was not straightforward. The sticking point was Metal shader compilation: the paths differ between iOS and macOS targets, and the build system was silently picking the wrong one for cross-compilation. This was the milestone that answered the fundamental question: will this run on-device across both platforms? Yes. Yes it will. With the GGUF engine wrapped behind InstrumentedEngineProtocol, the rest of the app doesn't know or care that there's a C++ inference runtime underneath — it's just another engine adapter that speaks Swift.