Add invader-hit SFX on bullet collisions in Space Invaders

[Copilot]

Space Invaders currently has no hit feedback when bullets destroy invaders, making successful shots feel muted. This PR adds a short, synthesized hit sound on bullet→invader collisions.

• Collision feedback wiring

  • Added hit-sound trigger to the existing bullet/alien overlap handler so each successful destroy event emits immediate audio feedback.

• Audio implementation

  • Introduced a small Web Audio helper that generates a brief retro-style descending square-wave blip (no asset files or new dependencies).
  • Reuses a lazily initialized AudioContext to avoid per-hit context creation.

• Runtime cleanup

  • Added oscillator.onended cleanup to disconnect temporary audio nodes after playback and prevent node buildup during rapid fire.

scene.physics.add.overlap(scene.si_bullets, scene.si_aliens, (bullet, alien) => {
  bullet.destroy();
  alien.destroy();
  _playInvaderHitSound();
});