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

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"
+    );
+}