Phase 2: multi-kill applause + umpire appreciation

A single shot that splatters ≥2 zombies now escalates: DOUBLE KILL /
TRIPLE KILL / MASSACRE ×N flash, louder crowd, and a new umpire line
(double_kill / triple_kill / unbelievable m4a, recorded say -v Daniel).
ball.kills tracks the streak, reset on every fresh shot (doHit + hitServe).
zombies.js imports cheer (now exported from gameplay). The splatter pace
cut is now TUNE.zPierce (0.75, was hardcoded 0.5) so a hot ball plows
through a lined-up lane instead of stalling on the first kill.

Verified via window.T: a low ball down a one-side lane kills 3→TRIPLE and
5→MASSACRE ×5 with escalating flash, streak resets on the next serve, the
three new lines serve 200 and match the existing m4a format, no crash when
the ball dies mid-streak, no console errors.

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

config.js

@@ -23,6 +23,7 @@ export const TUNE = {
   zSplatQ: 0.5,     // min shot quality to splatter; below = stagger pushback
   zPushback: 1.4,   // m a staggered zombie is knocked back
   splatterOverT: 0.1, // s the spent-ball 'over' beat lasts before the next serve is armed
+  zPierce: 0.75,    // pace a ball keeps after a splatter — hot balls plow through lined-up pairs
   missHud: 1,       // show the last miss reason on screen (full log: T.G.missLog)
 };