Sound: recorded SFX layer + a touch-easier CPU

Wire the new assets/audio/*.wav samples over the synth SFX:
- Ball hits: per-side tennis-player / tennis-opponent on serve & return,
  now with a stadium reverb tail (same convolver as umpire/aww).
- Crowd: tennis-applaus on your points/games (louder on big moments);
  tennis-aww when the opponent scores or you're eaten (reverb tail).
- Ambient bed: tennis-ambient-loop from first tap, 500Hz lowcut, low vol.
- Zombie music: zombie-mode loop when splatter kicks in (800Hz lowcut),
  fades out for the 1:1 sudden-death duel and back in after.
- Headshots: random headshot-1/-2 on every zombie kill (killZombie).

audio.js: sfx() one-shot player w/ rate+reverb opts, loopBed() beds with
duck/fade, aww() wrapper, hotter reverb send on the umpire voice.

Also: normal opponent aiFlub 0.09 -> 0.14 (a little easier to beat).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

main.js

@@ -7,7 +7,7 @@ import { TUNE, COURT, UMP_H, UMP_X } from './config.js';
 import { P, O, ball, score, G, setupPoint } from './state.js';
 import { update, keyPressed, solveShot, inServeBox, inCourt, forceZombies, splatPoint } from './gameplay.js';
 import { fitViewport, render, proj, RES } from './render.js';
-import { voice } from './audio.js';
+import { voice, ambient } from './audio.js';
 import { zombies, spawnZombie, piles, gore, parts, pellets, extras, explode } from './zombies.js';
 import { cv } from './dom.js';
 import './input.js';
@@ -27,7 +27,7 @@ setupPoint();
 const intro = document.getElementById('intro');
 function dismissIntro() {
   if (!intro.parentNode) return;
-  intro.remove(); voice('quiet_please');
+  intro.remove(); ambient(); voice('quiet_please'); // low atmosphere bed starts with the first gesture
 }
 intro.addEventListener('pointerdown', e => { e.preventDefault(); dismissIntro(); });
 addEventListener('keydown', () => dismissIntro(), { once: true });