Fix audio crash: use dummy SDL audio driver

--nosound flag doesn't exist in Spring binary. Instead, use environment
variables to force Spring to use dummy audio backends that don't require
hardware:
- SDL_AUDIODRIVER=dummy (SDL dummy audio backend)
- AUDIODEV=null (fallback)

Author: claude

.github/workflows/smoke-test.yml

@@ -159,9 +159,11 @@ jobs:
 
           # Run Spring with Xvfb (virtual X11 display) so LuaUI loads
           # This actually tests RmlUi initialization!
-          # Use --nosound to disable audio completely (no sound card in CI)
+          # Disable audio by removing any audio backend devices
+          export SDL_AUDIODRIVER=dummy
+          export AUDIODEV=null
           timeout 10s xvfb-run -a -s "-screen 0 1024x768x24" \
-            ./spring --nosound --write-dir $(pwd)/test-data script.txt || EXIT_CODE=$?
+            ./spring --write-dir $(pwd)/test-data script.txt || EXIT_CODE=$?
 
           # Wait a moment for Spring to finish writing logs
           sleep 2