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.

.github/workflows/studio-mac-api-smoke.yml

@@ -89,13 +89,8 @@ jobs:
           set -o pipefail
           bash install.sh --local --no-torch 2>&1 | tee logs/install.log
 
-      - name: Assert install.sh used the Mac llama.cpp prebuilt
-        run: |
-          if grep -q "falling back to source build" logs/install.log; then
-            echo "::error::install.sh fell back to source-build llama.cpp on Mac. Studio must install the prebuilt llama-bNNNN-bin-macos-arm64 on Apple Silicon."
-            grep -E "llama-prebuilt|llama.cpp" logs/install.log | tail -60
-            exit 1
-          fi
+      - name: Assert llama.cpp loads on this macOS
+        run: bash .github/scripts/assert-llama-loads.sh
 
       - name: Install pyjwt for the JWT-expiry forge test
         run: pip install 'pyjwt>=2.6'