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/constants.rs

@@ -47,3 +47,9 @@ pub const STAR_MAX_SIZE: f32 = 3.0;
 pub const STAR_MIN_SPEED: f32 = 20.0;
 pub const STAR_MAX_SPEED: f32 = 100.0;
 pub const STAR_Z_DEPTH: f32 = -10.0; // Behind all game entities
+
+// Explosion constants
+pub const EXPLOSION_DURATION: f32 = 0.4;
+pub const EXPLOSION_BASE_SIZE: Vec2 = Vec2::new(15.0, 15.0);
+pub const EXPLOSION_MAX_SIZE: Vec2 = Vec2::new(50.0, 50.0);
+pub const EXPLOSION_COLOR: Color = Color::rgba(1.0, 0.6, 0.1, 1.0);