ci: fix downstream integration - add submodules:recursive and pre-install metallib before build

Author: Aegis AI Assistant

.github/workflows/downstream_integration.yml

@@ -0,0 +1,64 @@
+name: Downstream Integration Test
+
+on:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  test-swiftlm-umbrella:
+    name: Test with SwiftLM (Umbrella Repo)
+    runs-on: macos-15
+    timeout-minutes: 40
+    steps:
+      - name: Checkout PR branch (mlx-swift)
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          path: mlx-swift-pr
+
+      - name: Checkout SwiftLM (Parent Repo)
+        uses: actions/checkout@v4
+        with:
+          repository: SharpAI/SwiftLM
+          path: SwiftLM
+          token: ${{ secrets.GITHUB_TOKEN }}
+          submodules: recursive
+
+      - name: Install Metal Toolchain
+        run: xcodebuild -downloadComponent MetalToolchain || true
+
+      - name: Resolve Dependencies (to populate SPM cache)
+        run: swift package resolve
+        working-directory: SwiftLM
+
+      - name: Override mlx-swift dependency with PR checkout
+        # SPM `package edit` puts a local editable copy of the package in
+        # .build/checkouts; we then replace its contents with our PR branch.
+        run: |
+          swift package edit mlx-swift --path $GITHUB_WORKSPACE/mlx-swift-pr
+        working-directory: SwiftLM
+
+      - name: Install MLX Metal library
+        run: |
+          python3 -m venv /tmp/mlx_venv
+          /tmp/mlx_venv/bin/pip install --quiet mlx
+          METALLIB=$(echo /tmp/mlx_venv/lib/python*/site-packages/mlx/lib/mlx.metallib)
+          # Make it available system-wide for all subsequent steps
+          sudo cp "$METALLIB" /usr/local/lib/mlx.metallib || true
+          echo "MLX_METAL_LIB=$METALLIB" >> $GITHUB_ENV
+
+      - name: Build SwiftLM
+        run: swift build --build-tests
+        working-directory: SwiftLM
+
+      - name: Copy Metal library to build outputs
+        run: |
+          METALLIB="$MLX_METAL_LIB"
+          # Copy to all locations the test runner and linked bundles may search
+          find SwiftLM/.build -type d \( -name "debug" -o -name "release" \) -exec cp "$METALLIB" {}/ \; 2>/dev/null || true
+          find SwiftLM/.build -name "*.xctest" -exec cp "$METALLIB" {}/Contents/MacOS/ \; 2>/dev/null || true
+          find SwiftLM/.build -name "*Tests" -type f -exec sh -c 'cp "'"$METALLIB"'" "$(dirname ${1})"/' _ {} \; 2>/dev/null || true
+
+      - name: Run SwiftLM Integration Tests
+        run: swift test --parallel --skip-build
+        working-directory: SwiftLM