feedback7: the battle sim uses engine-independent math — Chrome, Firefox, Safari and the server agree bit for bit (PR #373 by @Cloudnyco)

- server/sim/detmath.js: hypot (sqrt of the squares, rescaled outside 2^±500), powi (integer powers in double-double,
  one rounding), sin / cos (fdlibm 5.3) and atan2 (fdlibm / FreeBSD msun) built only from correctly rounded IEEE
  operations; the sim's Math.hypot / sin / cos / atan2 / pow calls go through it (65 files); eslint.config.js forbids
  the approximated Math functions under server/sim and test/sim/detmath.test.js scans for them.
- sin / cos / atan2 are the algorithms V8 runs in Node, so the server's results stay; only hypot differs from V8's.
  Golden: 2 moved — diy-027 (a hypot distance, damage +80) and coop3-ABYSS-5 (a later card pick from round 9, same end).
- Applied unchanged after review (Grok static review: MERGE; the diff read in full: no network, no new dependency).
- Tests: broad node --test 5155 / 5155 with the new file tracked (detmath 7 / 7, package 10 / 10); lint 0 errors,
  typecheck, check:imports.

server/sim/content/items/battle.js

@@ -47,6 +47,7 @@ import {
   alliesAround, passiveBuff, fxOn, battleStore, contentInfo, itemsOf, directMods,
 } from '../support/index.js';
 import { mitigate, hasHp, periodicDamage, isHpLoss } from '../../damage.js';
+import { hypot } from '../../detmath.js';
 
 // =====================================================================================================================
 // data helpers
@@ -465,7 +466,7 @@ const BY_ITEM = {
     const r = num(p.radius, 3), sc = num(p.damage_scale, 1);
     S.on('hit', (c) => {
       if (c.source !== u || !c.target || c.target.side !== 'enemy' || c.dmg.type === 'element') return;
-      if (Math.hypot(c.target.x - u.x, c.target.y - u.y) >= r - 1e-6) c.dmg.mul *= sc;
+      if (hypot(c.target.x - u.x, c.target.y - u.y) >= r - 1e-6) c.dmg.mul *= sc;
     });
   },
   // 炎国短刀: each skill activation +atk (≤ atk_buff_cnt stacks) — one 直接乘算 bonus of atk × stacks