Revert splatter rapid-fire serve; fix horde pose + re-trigger

Oliver's feedback on the feel pass:

1. Rapid-fire serving was way too easy. Reverted splatter serving to the
   proper model: a tap does the toss ritual (apex timing = quality), a
   flick is a quick aimed serve. Dropped the multi-ball barrage and
   removed the extraBalls plumbing (array, update, render, hooks). Melee
   swish and zombie homing stay. If multi-ball is wanted back, it should
   require a proper serve per ball.
2. The player was stuck in the serve/swing pose when a horde started
   (the triggering shot's pose lingered) — startHorde now clears it.
3. A horde re-triggered immediately: machine-gun kills scored points, so
   games were won mid-horde and the next kill after it ended won another
   game → instant new horde. A horde is now survival: it doesn't score
   (splatPoint skipped while G.horde), so it can't chain. After a horde
   you must win a full game again to summon the next.

Verified via window.T: a tap tosses (not an instant serve) and the full
toss→serve→rally completes; melee still kills adjacent shamblers; pose is
idle/-1 at horde start; no scoring during a horde and the next kill after
scores 15 without restarting it; tennis 11/12, bomb consume, homing all
still green; no console errors.

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

main.js

@@ -8,7 +8,7 @@ import { P, O, ball, score, G, setupPoint } from './state.js';
 import { update, keyPressed, solveShot, inServeBox, inCourt, forceZombies } from './gameplay.js';
 import { fitViewport, render, proj } from './render.js';
 import { voice } from './audio.js';
-import { zombies, spawnZombie, piles, gore, parts, pellets, extraBalls } from './zombies.js';
+import { zombies, spawnZombie, piles, gore, parts, pellets } from './zombies.js';
 import { cv } from './dom.js';
 import './input.js';
 
@@ -54,6 +54,6 @@ cv.addEventListener('pointerdown', e => {
 requestAnimationFrame(frame);
 
 // exposed for in-browser sanity checks and simulated playtests
-window.T = { TUNE, ball, P, O, score, G, zombies, spawnZombie, piles, gore, parts, pellets, extraBalls, solveShot, inServeBox, inCourt, proj, setupPoint, forceZombies,
+window.T = { TUNE, ball, P, O, score, G, zombies, spawnZombie, piles, gore, parts, pellets, solveShot, inServeBox, inCourt, proj, setupPoint, forceZombies,
   step: dt => update(dt), render, press: keyPressed,
   get phase() { return G.phase; }, set phase(v) { G.phase = v; } };