feat(game): add special stage type every 3rd level

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

src/constants.rs

@@ -47,6 +47,9 @@ pub const BEAM_CORE_COLOR: Color = Color::srgba(0.7, 0.2, 1.0, 0.7);
 pub const BEAM_PULSE_FREQ: f32 = 3.0;
 pub const BEAM_PULSE_AMPLITUDE: f32 = 0.15;
 
+// Special stages
+pub const SPECIAL_STAGE_INTERVAL: u32 = 3;
+
 // Starfield
 pub const STAR_COUNT: usize = 150;
 pub const STAR_MIN_SIZE: f32 = 1.0;