Add macOS 15 CI target and fix ARM i8mm feature conflict

On macOS 15 (Apple Clang 16), stricter always_inline enforcement causes a
build failure when llama.cpp generates -mcpu=native+dotprod+noi8mm flags
while code includes i8mm-requiring always_inline functions. Adding
-DGGML_NATIVE=OFF for the macos-15 runner disables native CPU feature
detection, avoiding the contradictory i8mm enable/disable state.

Also use runner-specific artifact names to avoid upload conflicts when
multiple macOS matrix jobs fail simultaneously.

https://claude.ai/code/session_01PjfeKeUZE8spcY4atNbymE

Author: claude

.github/workflows/ci.yml

@@ -49,6 +49,8 @@ jobs:
             cmake: -DLLAMA_METAL=OFF -DLLAMA_VERBOSE=ON
           - runner: macos-14
             cmake: -DLLAMA_METAL_EMBED_LIBRARY=ON -DLLAMA_VERBOSE=ON
+          - runner: macos-15
+            cmake: -DLLAMA_METAL_EMBED_LIBRARY=ON -DLLAMA_VERBOSE=ON -DGGML_NATIVE=OFF
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-java@v4
@@ -70,7 +72,7 @@ jobs:
       - if: failure()
         uses: actions/upload-artifact@v4
         with:
-          name: error-log-macos
+          name: error-log-${{ matrix.target.runner }}
           path: ${{ github.workspace }}/hs_err_pid*.log
           if-no-files-found: warn