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/kits/ops/chess_char_4_20-fartth.js

@@ -6,6 +6,7 @@ import { bodyInKeys } from '../../../body.js';
 import { absoluteRangeKeys, sortEnemyTargets } from '../../../targeting.js';
 import { isHpLoss } from '../../../damage.js';
 import { num, tbb, whileDeployed, toggleBuff, skillActive, isSel, alt, withDefaults } from '../shared/tier4.js';
+import { hypot } from '../../../detmath.js';
 
 const LINE = Object.freeze(Array.from({ length: COLS }, (_, i) => Object.freeze([0, i])));
 const tileKey = (u) => Math.round(u.y) * COLS + Math.round(u.x);
@@ -83,7 +84,7 @@ export default withDefaults({
         battle.on('hit', (c) => {
           if (c.source !== unit || !c.dmg.isAttack) return;
           if (S3 && skillActive(unit) && unit.baseRangeKeys && !bodyInKeys(c.target, unit.baseRangeKeys)) c.dmg.mul *= num(bb.damage_scale, 1.25);
-          if (ds > 0) { const d = Math.hypot(c.target.x - unit.x, c.target.y - unit.y); c.dmg.mul *= 1 + ds * Math.max(0, Math.min(1, (d - lo) / Math.max(1e-6, hi - lo))); }
+          if (ds > 0) { const d = hypot(c.target.x - unit.x, c.target.y - unit.y); c.dmg.mul *= 1 + ds * Math.max(0, Math.min(1, (d - lo) / Math.max(1e-6, hi - lo))); }
         }, { owner: unit });
       },
     };