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/enemy.rs

@@ -12,6 +12,7 @@ use crate::constants::{
 };
 use crate::resources::{
     AttackDiveTimer, CurrentStage, EnemySpawnTimer, FormationState, StageConfigurations,
+    is_special_stage,
 };
 
 const BOSS_BASE_CHANCE: f32 = 0.30;
@@ -341,7 +342,11 @@ pub fn enemy_shoot(
     mut commands: Commands,
     time: Res<Time>,
     mut enemy_query: Query<(&Transform, &mut Enemy, &EnemyState), Without<FormationTarget>>,
+    stage: Res<CurrentStage>,
 ) {
+    if is_special_stage(stage.number) {
+        return;
+    }
     for (transform, mut enemy, state) in enemy_query.iter_mut() {
         if !matches!(state, EnemyState::Attacking(_)) {
             continue;