Phase 6: racket zombies (rally mini-boss)

Survive a horde and a racket zombie takes up residence on the far court
(variant 'r', one per horde survived) — the tennis rally skill lives on
inside zombie mode. It reuses the turned-CPU z_c sprite (already holds a
racket) and a slimmed copy of the tennis AI: holds station deep, chases a
ball laterally at TUNE.racketSpeed (2.8), and returns your SOFT shots
(pow < zSplatQ) as live lastHit:'O' balls with quality capped at
racketQMax (0.55) and racketFlub (0.2) whiffs. trySwing gained an
optional quality-cap arg (defaulted, so tennis is untouched).

It's a mini-boss with TUNE.racketHp (3): only splat-quality balls hurt it
(-1 hp, staggers the ball; the killing blow runs the full splatter). Soft
balls pass through for it to rally; melee and pellets don't touch it. Its
return that gets past your baseline is a point for THEM; you can rally it
back with the existing player hit. A red HP bar floats over its head.

Verified via window.T: endHorde spawns exactly one (hp 3, z 3.5); three
splat hits kill it (2→1→dead); a pow-0.3 ball is returned toward you; an
'O' ball past your baseline scores for THEM; melee spares the boss but
cleaves the shambler beside it; tennis still 12/11/12 and a normal
shambler is still one-shot; renders with its HP bar; no console errors.

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

config.js

@@ -31,6 +31,10 @@ export const TUNE = {
   hordeRate: 2.5,   // zombies spawned per second during a horde
   mgRate: 6,        // machine-gun pellets fired per second during a horde
   meleeRange: 1.7,  // m — a racket swish cleaves shamblers this close to you
+  racketHp: 3,      // splat-quality hits a racket zombie (mini-boss) takes before it dies
+  racketSpeed: 2.8, // m/s the racket zombie shambles to chase a ball
+  racketQMax: 0.55, // its return quality is capped here (slower, sloppier than the tennis CPU)
+  racketFlub: 0.2,  // chance the racket zombie whiffs a return
   missHud: 1,       // show the last miss reason on screen (full log: T.G.missLog)
 };