Fix C++ compatibility workflow: build spine-libgdx first

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Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/spine-cpp-compatibility.yml

@@ -35,6 +35,10 @@ jobs:
         key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
         restore-keys: ${{ runner.os }}-gradle
 
+    - name: Build spine-libgdx first
+      working-directory: spine-libgdx
+      run: ./gradlew build
+
     - name: Run C++ vs Java compatibility test
       working-directory: tests
       run: ./test.sh cpp -s spineboy -f