Every 3rd stage (3, 6, 9, ...) is now a special stage with no enemy
dive attacks and no enemy shooting. Enemies still spawn in the default
grid formation (32 enemies) but remain in formation without attacking.
Changes:
- Add SPECIAL_STAGE_INTERVAL constant and is_special_stage() function
- Add special_stage config to StageConfigurations with empty attack_patterns
- Modify for_stage() to route special stages to special config
- Guard enemy_shoot system to skip shooting during special stages
- Show '*' suffix in window title for special stages
Refs: GAL-39
