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;

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;

src/resources.rs

@@ -3,9 +3,14 @@ use bevy::prelude::*;
 use crate::components::AttackPattern;
 use crate::constants::{
     ENEMY_SHOOT_INTERVAL, FORMATION_BASE_Y, FORMATION_COLS, FORMATION_ENEMY_COUNT,
-    FORMATION_X_SPACING, FORMATION_Y_SPACING, WINDOW_WIDTH,
+    FORMATION_X_SPACING, FORMATION_Y_SPACING, SPECIAL_STAGE_INTERVAL, WINDOW_WIDTH,
 };
 
+/// Returns `true` when `stage_number` is a special stage (every 3rd stage).
+pub fn is_special_stage(stage_number: u32) -> bool {
+    stage_number > 0 && stage_number.is_multiple_of(SPECIAL_STAGE_INTERVAL)
+}
+
 #[derive(Resource)]
 pub struct EnemySpawnTimer {
     pub timer: Timer,
@@ -58,13 +63,21 @@ pub struct StageConfig {
 #[derive(Resource, Debug, Clone)]
 pub struct StageConfigurations {
     pub stages: Vec<StageConfig>,
+    pub special_stage: StageConfig,
 }
 
 impl StageConfigurations {
-    /// Cycles through configured stages once `stage_number` exceeds the count.
+    /// Returns the config for the given stage, using the special stage config
+    /// when `stage_number` is a multiple of `SPECIAL_STAGE_INTERVAL`.
     pub fn for_stage(&self, stage_number: u32) -> &StageConfig {
-        let idx = (stage_number.saturating_sub(1) as usize) % self.stages.len();
-        &self.stages[idx]
+        if is_special_stage(stage_number) {
+            return &self.special_stage;
+        }
+        // Count how many special stages come before this one
+        let special_before = stage_number / SPECIAL_STAGE_INTERVAL;
+        let normal_idx =
+            (stage_number.saturating_sub(1) - special_before) as usize % self.stages.len();
+        &self.stages[normal_idx]
     }
 }
 
@@ -89,8 +102,18 @@ impl Default for StageConfigurations {
             enemy_shoot_interval: ENEMY_SHOOT_INTERVAL * 0.8,
         };
 
+        let special_stage = StageConfig {
+            formation_layout: FormationLayout::default(),
+            enemy_count: FORMATION_ENEMY_COUNT,
+            attack_patterns: vec![],
+            attack_dive_interval: f32::MAX,
+            enemy_speed_multiplier: 1.0,
+            enemy_shoot_interval: ENEMY_SHOOT_INTERVAL,
+        };
+
         Self {
             stages: vec![stage1, stage2],
+            special_stage,
         }
     }
 }

src/systems.rs

@@ -4,7 +4,7 @@ use std::time::Duration;
 use crate::components::{Explosion, Invincible, Player};
 use crate::constants::{EXPLOSION_BASE_SIZE, EXPLOSION_COLOR, EXPLOSION_DURATION, EXPLOSION_MAX_SIZE};
 use crate::player::spawn_player_ship;
-use crate::resources::{CurrentStage, PlayerLives, Score};
+use crate::resources::{CurrentStage, PlayerLives, Score, is_special_stage};
 use crate::starfield::spawn_starfield;
 
 pub fn setup(mut commands: Commands) {
@@ -38,10 +38,15 @@ pub fn update_window_title(
     if !(lives.is_changed() || score.is_changed() || stage.is_changed()) {
         return;
     }
+    let stage_label = if is_special_stage(stage.number) {
+        format!("{}*", stage.number)
+    } else {
+        stage.number.to_string()
+    };
     if let Ok(mut window) = windows.single_mut() {
         window.title = format!(
             "Galaga :: Stage: {} Lives: {} Score: {}",
-            stage.number, lives.count, score.value
+            stage_label, lives.count, score.value
         );
     }
 }

tests/special_stage.rs

@@ -0,0 +1,140 @@
+use bglga::constants::SPECIAL_STAGE_INTERVAL;
+use bglga::resources::is_special_stage;
+use bglga::resources::StageConfigurations;
+
+// ── SPECIAL_STAGE_INTERVAL constant ──
+
+#[test]
+fn special_stage_interval_is_three() {
+    assert_eq!(SPECIAL_STAGE_INTERVAL, 3);
+}
+
+// ── is_special_stage function ──
+
+#[test]
+fn stage_zero_is_not_special() {
+    assert!(!is_special_stage(0));
+}
+
+#[test]
+fn stage_1_is_not_special() {
+    assert!(!is_special_stage(1));
+}
+
+#[test]
+fn stage_2_is_not_special() {
+    assert!(!is_special_stage(2));
+}
+
+#[test]
+fn stage_3_is_special() {
+    assert!(is_special_stage(3));
+}
+
+#[test]
+fn stage_4_is_not_special() {
+    assert!(!is_special_stage(4));
+}
+
+#[test]
+fn stage_5_is_not_special() {
+    assert!(!is_special_stage(5));
+}
+
+#[test]
+fn stage_6_is_special() {
+    assert!(is_special_stage(6));
+}
+
+#[test]
+fn stage_9_is_special() {
+    assert!(is_special_stage(9));
+}
+
+// ── for_stage returns special config for special stages ──
+
+#[test]
+fn for_stage_returns_special_config_at_stage_3() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(3);
+    // Special stages have no dive attacks
+    assert!(
+        config.attack_patterns.is_empty(),
+        "Stage 3 (special) should have empty attack_patterns"
+    );
+}
+
+#[test]
+fn for_stage_returns_special_config_at_stage_6() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(6);
+    assert!(
+        config.attack_patterns.is_empty(),
+        "Stage 6 (special) should have empty attack_patterns"
+    );
+}
+
+#[test]
+fn for_stage_returns_normal_config_at_stage_1() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(1);
+    assert!(
+        !config.attack_patterns.is_empty(),
+        "Stage 1 (normal) should have attack patterns"
+    );
+}
+
+#[test]
+fn for_stage_returns_normal_config_at_stage_2() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(2);
+    assert!(
+        !config.attack_patterns.is_empty(),
+        "Stage 2 (normal) should have attack patterns"
+    );
+}
+
+#[test]
+fn for_stage_returns_normal_config_at_stage_4() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(4);
+    assert!(
+        !config.attack_patterns.is_empty(),
+        "Stage 4 (normal) should have attack patterns"
+    );
+}
+
+// ── Special stage config properties ──
+
+#[test]
+fn special_stage_has_full_formation() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(3);
+    assert_eq!(config.enemy_count, 32, "Special stage should have 32 enemies");
+    assert_eq!(
+        config.formation_layout.positions.len(),
+        32,
+        "Special stage formation should have 32 positions"
+    );
+}
+
+#[test]
+fn special_stage_has_max_dive_interval() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(3);
+    assert_eq!(
+        config.attack_dive_interval,
+        f32::MAX,
+        "Special stage should have max dive interval"
+    );
+}
+
+#[test]
+fn special_stage_has_base_speed() {
+    let configs = StageConfigurations::default();
+    let config = configs.for_stage(3);
+    assert_eq!(
+        config.enemy_speed_multiplier, 1.0,
+        "Special stage should have 1.0x speed multiplier"
+    );
+}