feat: add explosion animations on entity destruction

Spawn expanding/fading orange explosion effects when enemies or
the player are destroyed. Explosions scale from 15x15 to 50x50
over 0.4s while fading from full opacity to transparent, then
auto-despawn.

Integration points:
- Enemy killed by player bullet (bullet.rs)
- Player hit by enemy bullet (bullet.rs)
- Player collides with enemy (player.rs) - both explode
- Captured player released (player.rs)

Refs: GAL-44

src/main.rs

@@ -36,7 +36,7 @@ use bullet::{
     move_enemy_bullets,
 };
 use stage::check_stage_clear;
-use systems::{player_exists, player_vulnerable, setup, should_respawn_player, update_window_title};
+use systems::{player_exists, player_vulnerable, setup, should_respawn_player, update_window_title, animate_explosion};
 use starfield::scroll_starfield;
 
 fn main() {
@@ -119,6 +119,8 @@ fn main() {
         )
         // Starfield runs in all states
         .add_systems(Update, scroll_starfield)
+        // Explosion animation runs in all states
+        .add_systems(Update, animate_explosion)
         // UI and state management systems
         .add_systems(OnEnter(AppState::StartMenu), setup_start_menu_ui)
         .add_systems(OnExit(AppState::StartMenu), cleanup_start_menu_ui)