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).
