ci(engine-launcher): install xvfb for headless window-opening smoke tests

smoke.sh already wraps with xvfb-run when DISPLAY is unset, but xvfb
package wasn't installed in the runner. Window-opening tests (S4,
E.3-S1) segfaulted on raylib InitWindow without X server.

.gitea/workflows/reusable-engine-launcher.yml

@@ -40,6 +40,8 @@ jobs:
             libx11-dev libxrandr-dev libxi-dev libxcursor-dev libxinerama-dev \
             libxkbcommon-dev libwayland-dev \
             libgl1-mesa-dev libglu1-mesa-dev
+          # xvfb: headless X server for window-opening smoke tests
+          apt-get install -y xvfb
 
       - name: Configure CMake
         run: cmake -B build -DCMAKE_BUILD_TYPE=${{ inputs.build_type }}