Phase 11: enemy roster wired — runner, brute, shield zombie

- Spawn mix (pickVariant): roster grows one type per set — runner from set 1
  game 3+ (12% slot, at most one alive, shriek on spawn = the fairness contract),
  brute from set 2 (15%), shield from set 3 (15%); classics fill the rest
- Runner ('f'): 3.2x shamble speed, faster leg cycle; new shriek() synth in audio
- Brute ('g'): hp 2 vs splat-quality balls AND pellets, 0.55x speed, 0.75m target,
  pushback-immune, 0.9m eaten radius, dies to double gore + size-2 pile, towers
  at 1.5x height
- Shield ('s'): blocks front balls with the chair (clank, ball spent), the block
  drops the chair for shieldDown 1.5s — any splat-quality ball kills in the
  window (z_s3 staggered pose renders); chair recovers after; always dies to
  bombs, melee, pellets
- TUNE: runnerSpeed 3.2, bruteHp 2, shieldDown 1.5

Verified in browser: per-variant stats; brute soft-ball immunity, 2-hit kill,
2-pellet kill, size-2 pile; shield block->window kill->chair recovery->blocks
again; melee kills f/s but not the racket boss; tier-2 spawn mix over 1500
spawns = 14/15/8/63%; all five sprites render with correct scale+occlusion;
tennis regression 11/12; no console errors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

audio.js

@@ -116,6 +116,21 @@ export function boom(vol = 0.9) { // bomb ball — sub-bass thump under a filter
   } catch (e) {}
 }
 
+export function shriek(vol = 0.35) { // a runner announces itself — harsh rising cry, the fairness contract
+  try {
+    const ac = audio(), t = ac.currentTime, dur = 0.7;
+    const o = ac.createOscillator(); o.type = 'sawtooth';
+    o.frequency.setValueAtTime(300, t);
+    o.frequency.exponentialRampToValueAtTime(950, t + 0.25);
+    o.frequency.exponentialRampToValueAtTime(500, t + dur);
+    const bp = ac.createBiquadFilter(); bp.type = 'bandpass'; bp.frequency.value = 1400; bp.Q.value = 0.8;
+    const g = ac.createGain();
+    g.gain.setValueAtTime(0.001, t); g.gain.exponentialRampToValueAtTime(vol, t + 0.06);
+    g.gain.exponentialRampToValueAtTime(0.001, t + dur);
+    o.connect(bp).connect(g).connect(ac.destination); o.start(t); o.stop(t + dur + 0.05);
+  } 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;