feedback5 CB2: persistent ground areas stay readable when they stack — a zone re-sent under one key updates in place, persistent areas are dimmer and share their light where they overlap (community report of 2026-10-06 「炼金单元等范围持续性技能特效太亮（尤其是多层叠加以后）」)

Cause: 引星棘刺 S2's drifting alchemy unit is re-sent every second (position and radius change) and the client drew each
event as a new additive zone lasting the rest of the unit's life — up to twelve layers on one spot; overlapping units
(锡人's charges, several alchemists) added up the same way (render/fx/zones.js: disc alpha 0.28, edge 0.75, ADD blend).
Now: the sim fx carries `key` (kits/ops/chess_char_5_15-thorn2.js) and zones.js updates the zone under that key in place;
zoneAlpha draws a persistent area (> 3 real s) at 0.16 / 0.5 and ÷ √n among the n persistent areas overlapping it (centres
closer than ¾ of their radii summed); short bursts and telegraphs unchanged.

Golden: unchanged (fx events are counted by kind; the snapshots do not carry fx).
Tests: test/render/feedback5-zone-stack.test.js (3, fail on the parent).

test/render/feedback5-zone-stack.test.js

@@ -0,0 +1,75 @@
+// test/render/feedback5-zone-stack.test.js — persistent ground areas (炼金单元 …) stay readable when they stack (community
+// report of 2026-10-06 「炼金师分支干员的炼金单元等范围持续性技能特效太亮（尤其是多层叠加以后）」): a zone the sim re-sends under one
+// `key` (引星棘刺 S2's drifting unit, every second) is updated in place instead of piling up a new additive layer each time,
+// and a persistent area is drawn dimmer and shares its light with the persistent areas overlapping it (÷ √n) —
+// render/fx/zones.js zone / zoneAlpha, render/fx/simfx.js (the event's `key`).
+// Run: node --test test/render/feedback5-zone-stack.test.js
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { FxSystem } from '../../public/js/render/fx.js';
+import { zoneAlpha, ZONE_ALPHA, ZONE_PERSIST } from '../../public/js/render/fx/zones.js';
+import { presetCamera } from '../../public/js/render/projection.js';
+
+/** An FxSystem shell with fake sprites: enough for zone() / _updateZones() / simFx('zone'). */
+function fxShell() {
+  const cam = presetCamera('normal', { width: 1280, height: 720 });
+  class Sprite {
+    constructor() { this.anchor = { set() {} }; this.position = { set() {} }; this.scale = { set() {} }; this.alpha = 1; this.tint = 0; this.destroyed = false; }
+    destroy() { this.destroyed = true; }
+  }
+  const fx = Object.create(FxSystem.prototype);
+  Object.assign(fx, {
+    P: { Sprite, BLEND_MODES: { ADD: 1 } },
+    tex: { soft: {}, ring: {} },
+    zones: [],
+    ctx: { cam: () => cam, heightAt: () => 0, timeScale: () => 2, view: () => null },
+    _p: { x: 0, y: 0, s: 0, depth: 0 }, _q: { x: 0, y: 0, s: 0, depth: 0 },
+    _onGround() {},
+    _groundZ: () => 0,
+  });
+  return fx;
+}
+
+test('a zone re-sent under the same key is moved / resized in place — one layer, not one per event', () => {
+  const fx = fxShell();
+  fx.simFx('zone', 5, 10, { id: 1, r: 1.1, duration: 12, key: 'thorn2:7:1' });
+  for (let i = 1; i <= 11; i++) {
+    fx._updateZones(0.5);
+    fx.simFx('zone', 5 + 0.1 * i, 10, { id: 1, r: 1.1 + 0.13 * i, duration: 12 - i, key: 'thorn2:7:1' });
+  }
+  assert.equal(fx.zones.length, 1, 'twelve events, one zone');
+  const zn = fx.zones[0];
+  assert.ok(Math.abs(zn.x - 6.1) < 1e-9 && Math.abs(zn.r - (1.1 + 0.13 * 11)) < 1e-9, 'it follows the last event (drift, growth)');
+  assert.ok(Math.abs(zn.dur - (zn.t + Math.max(0.6, 1 / 2))) < 1e-9, 'its end: the remaining game second at 2× (at least 0.6 s) from now');
+  fx.simFx('zone', 8, 10, { id: 1, r: 1, duration: 12, key: 'thorn2:7:2' });
+  fx.simFx('zone', 8, 11, { id: 2, r: 2, duration: 11 });
+  fx.simFx('zone', 8, 11, { id: 2, r: 2, duration: 11 });
+  assert.equal(fx.zones.length, 4, 'another key, and unkeyed zones (锡人\'s units, one event each), are zones of their own');
+});
+
+test('persistent areas are dimmer and share their light when they overlap; short bursts and telegraphs keep theirs', () => {
+  const zone = (x, dur = 6, r = 2, warn = false) => ({ x, y: 10, r, dur, warn });
+  const alone = zone(2);
+  assert.deepEqual(zoneAlpha(alone, [alone]), ZONE_ALPHA.persist);
+  const stack = [zone(5), zone(5.2), zone(5.4), zone(5.6)];
+  const [disc, edge] = zoneAlpha(stack[0], stack);
+  assert.ok(Math.abs(disc - ZONE_ALPHA.persist[0] / 2) < 1e-12 && Math.abs(edge - ZONE_ALPHA.persist[1] / 2) < 1e-12, 'four on one spot: ÷ √4 each');
+  assert.ok(4 * disc <= 2 * ZONE_ALPHA.short[0], `the four discs together (${(4 * disc).toFixed(2)}) stay within two short bursts`);
+  const far = [zone(2), zone(9)];
+  assert.deepEqual(zoneAlpha(far[0], far), ZONE_ALPHA.persist, 'apart: no damping');
+  const short = zone(5, ZONE_PERSIST - 0.5);
+  assert.deepEqual(zoneAlpha(short, [short, ...stack]), ZONE_ALPHA.short, 'a short burst keeps its strength');
+  const warn = zone(5, 6, 2, true);
+  assert.deepEqual(zoneAlpha(warn, [warn, ...stack]), ZONE_ALPHA.warn, 'a telegraph keeps its strength');
+});
+
+test('the renderer applies it: four alchemy units on one spot draw their discs at half the lone strength', () => {
+  const fx = fxShell();
+  for (let i = 0; i < 4; i++) fx.simFx('zone', 5 + 0.1 * i, 10, { id: 9, r: 2, duration: 11 });
+  fx.simFx('zone', 15, 10, { id: 9, r: 2, duration: 11 });
+  fx._updateZones(1); // past the 0.25 s growth, before the fade
+  const lone = fx.zones[4], stacked = fx.zones.slice(0, 4);
+  const ratio = stacked[0].disc.alpha / lone.disc.alpha;
+  assert.ok(Math.abs(ratio - 0.5) < 1e-9, `÷ √4 (${ratio})`);
+});