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>

config.js

@@ -14,7 +14,7 @@ export const TUNE = {
   serveSpeed: 21, serveQualitySpeed: 7,
   aimError: 2.4,           // m of scatter at quality 0
   perfectMs: 45,           // |offset from window center| for PERFECT
-  aiQualityMin: 0.45, aiQualityMax: 0.85, aiFlub: 0.09,
+  aiQualityMin: 0.45, aiQualityMax: 0.85, aiFlub: 0.14, // ~86% of returns land (was 0.09) — a touch easier to beat
   aiReaction: 0.45,        // s before the CPU reads your shot — well-aimed corners are winners
   autoAssist: matchMedia('(pointer: coarse)').matches ? 1.15 : 0, // auto-run speed factor (mobile)
   zSpeed: 0.7,      // zombie shamble speed m/s