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).

public/js/render/fx/simfx.js

@@ -84,7 +84,8 @@ export class FxSim {
         this.mortar(this._viewOf(ex.id ?? ex.src), at.x, at.y, r, flight);
         break;
       }
-      case 'zone': this.zone(at.x, at.y, at.z, r, col, Math.max(0.6, dur || 1.5), spec.tex); break;
+      // `key`: one zone the sim re-sends as it moves / grows (an update in place, never a second layer — zones.js)
+      case 'zone': this.zone(at.x, at.y, at.z, r, col, Math.max(0.6, dur || 1.5), spec.tex, false, ex.key != null ? String(ex.key) : null); break;
       case 'wall': {
         // a line of burning tiles through the anchor tile along `axis` ('col' | 'row', from the sim event)
         const rect = this.ctx.fieldRect ? this.ctx.fieldRect() : null;