Replace the single static rectangle with a 2-layer beam (outer glow +
inner core) and sinusoidal opacity pulse. Add per-frame beam height
tracking to follow boss position. Include 8 unit tests for pure math
functions (beam height calculation, pulse alpha).
Refs: GAL-33
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
Add 150 stars with randomized positions, sizes, speeds, and brightness.
Stars scroll downward with parallax depth effect and wrap around at
screen edges. Also fixes Bevy 0.13 API issues in game_state.rs
(KeyCode::R → KeyR, Input → ButtonInput).
