studio: pick a macOS llama.cpp prebuilt that loads on the host OS (#5883)

Make macOS llama.cpp prebuilt selection host-OS-version aware: skip a prebuilt whose minimum-OS exceeds the host and walk back to the newest release that loads (macOS 26 keeps latest; 14/15 land on a compatible older release). Source-build fallback pins CMAKE_OSX_DEPLOYMENT_TARGET=13.3. CI: binary-load assertion plus a macos-14/15/26 install matrix. No change to Linux/Windows or CUDA selection.