App.run ended its menu loop on Quit and just returned. The console exits then,
but in the GUI run() is on a daemon worker while the Swing JFrame stays
displayable on the EDT, keeping the JVM alive — and the dismissed menu overlay
just revealed the running game underneath ("back into the game"). EXIT_ON_CLOSE
only fires for the native window X, not a programmatic quit.
Add a GameIO.shutdown() teardown hook (default no-op) and call it after the menu
loop ends. SwingIO overrides it to stop music, dispose the window, and exit the
JVM, mirroring EXIT_ON_CLOSE; console keeps the no-op. Extract an injectable
App.run overload and add AppQuitTest asserting the Quit path invokes shutdown().
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
