Add Lissajous and cubic Bezier path types for enemies on special stages
(every 3rd level). Path-following enemies spawn with ScriptedPath marker
and FollowingPath state, move along parametric curves, and despawn when
their path ends off-screen.
Key changes:
- New flight_paths module with pure path evaluation (Lissajous + Bezier)
- ScriptedPath marker component and FollowingPath EnemyState variant
- flight_patterns field on StageConfigurations for composable path data
- Special stage spawn branch in spawn_enemies() with stagger delays
- Path-following movement in move_enemies() with delay gating and despawn
- Formation complete early return for special stages
- Without<ScriptedPath> filter on attacking_query to prevent double-processing
- 13 new unit tests for path evaluation and data contracts
Refs: GAL-40
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
