Phase 4: bomb balls

Ball boys deliver a bomb ball every TUNE.bombEvery (8) splatter kills, up
to TUNE.bombMax (3) in stock (G.bombs / G.killMeter in state). The next
serve auto-consumes one (hitServe, splatter-only) — no extra input, keeping
one-handed play. A bomb detonates on first zombie contact (regardless of
pace) or, if it misses everyone, on its first bounce: explode() splatters
everything within TUNE.bombRadius (2.5m), each kill feeding ball.kills so
the Phase-2 fanfare escalates naturally, and leaves a crater — big stain,
size-3 pile, 40 particles, 0.1s hitstop, boom(). New synth boom() in
audio.js. Bomb ball renders black with a flickering orange fuse; the HUD
shows stocked bombs as black/orange pips by the YOU line. Both explode
sites set phase='over' so the serve loop doesn't stall on the killed ball.

Alternative delivery ideas Oliver floated but not built: umpire throws it,
or court pickup.

Verified via window.T: 8th kill earns a bomb (meter resets); a serve
consumes one and renders #111 black + orange fuse; a pow-0 bomb amid 4
clustered zombies kills all 4 with 'MASSACRE ×4', boom, hitstop 0.1,
crater stain (r1.4) + size-4 pile; a bomb that misses still craters on
first bounce; bombs never leak into tennis (mode guard); 3 HUD pips; no
console errors.

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

audio.js

@@ -100,6 +100,22 @@ export function splat(vol = 0.7) { // wet burst — a zombie meets a first serve
   } catch (e) {}
 }
 
+export function boom(vol = 0.9) { // bomb ball — sub-bass thump under a filtered noise blast
+  try {
+    const ac = audio(), t = ac.currentTime;
+    const o = ac.createOscillator(), og = ac.createGain(); // 80Hz sine diving to 30Hz
+    o.frequency.setValueAtTime(80, t); o.frequency.exponentialRampToValueAtTime(30, t + 0.35);
+    og.gain.setValueAtTime(vol, t); og.gain.exponentialRampToValueAtTime(0.001, t + 0.4);
+    o.connect(og).connect(ac.destination); o.start(t); o.stop(t + 0.42);
+    const src = ac.createBufferSource(); src.buffer = noiseBuf(ac, 0.4); // the blast
+    const lp = ac.createBiquadFilter(); lp.type = 'lowpass';
+    lp.frequency.setValueAtTime(2200, t); lp.frequency.exponentialRampToValueAtTime(200, t + 0.35);
+    const g = ac.createGain();
+    g.gain.setValueAtTime(vol * 0.8, t); g.gain.exponentialRampToValueAtTime(0.001, t + 0.4);
+    src.connect(lp).connect(g).connect(ac.destination); src.start(t);
+  } catch (e) {}
+}
+
 export function moan(vol = 0.1) { // distant undead ambience
   try {
     const ac = audio(), t = ac.currentTime, dur = 1.2 + Math.random() * 0.8;