feat: improve tractor beam visual with 2-layer glow and pulse animation

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

src/main.rs

@@ -29,7 +29,7 @@ use player::{
 };
 use enemy::{
     check_formation_complete, enemy_shoot, is_formation_complete, move_enemies, spawn_enemies,
-    trigger_attack_dives, boss_capture_attack,
+    trigger_attack_dives, boss_capture_attack, update_tractor_beam_visual,
 };
 use bullet::{
     check_bullet_collisions, check_enemy_bullet_player_collisions, move_bullets,
@@ -103,6 +103,7 @@ fn main() {
                 move_enemies,
                 enemy_shoot, // Consider run_if attacking state? (Handled internally for now)
                 boss_capture_attack, // New system for boss tractor beam
+                update_tractor_beam_visual,
             )
                 .run_if(in_state(AppState::Playing)),
         )