SessionManager.new_game() now returns "" on dir creation failure
instead of proceeding with a broken game-id. Main menu guards
against empty return. Test suite tracks and cleans up created
save directories in after_test().
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Per-game save directories under user://saves/<timestamp>-<seed>/.
SessionManager autoload handles new_game(), resume_game(), quit flow.
Main menu scene with New Game / Continue / Quit buttons. Game-id
passed to server subprocess via --game-id flag.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>