Add C++ runtime compatibility test workflow

Verifies C++ runtime produces identical skeleton data output to Java reference implementation.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

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

@@ -0,0 +1,48 @@
+name: C++ Runtime Compatibility Test
+
+on:
+  push:
+  workflow_dispatch:
+
+jobs:
+  test-cpp-java-compatibility:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - name: Set up JDK 17
+      uses: actions/setup-java@v4
+      with:
+        java-version: '17'
+        distribution: 'temurin'
+
+    - name: Set up Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version: '18'
+
+    - name: Install build dependencies
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y build-essential cmake
+
+    - name: Cache Gradle packages
+      uses: actions/cache@v4
+      with:
+        path: ~/.gradle/caches
+        key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*') }}
+        restore-keys: ${{ runner.os }}-gradle
+
+    - name: Run C++ vs Java compatibility test
+      working-directory: tests
+      run: ./test.sh cpp -s spineboy -f
+
+    - name: Upload test artifacts on failure
+      if: failure()
+      uses: actions/upload-artifact@v4
+      with:
+        name: test-output-files
+        path: tests/output/
+        retention-days: 7