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/projectiles.js

@@ -5,6 +5,7 @@
 // mid-flight the projectile fizzles, unless `hitDead: true` (then it lands at the last known position).
 
 import { PROJECTILE_SPEED } from './constants.js';
+import { hypot } from './detmath.js';
 
 let seq = 0;
 const fin = (v, d) => (typeof v === 'number' && Number.isFinite(v) ? v : d);
@@ -55,7 +56,7 @@ export class ProjectileSystem {
         else p.target = null;
       }
       const dx = p.tx - p.x, dy = p.ty - p.y;
-      const d = Math.hypot(dx, dy);
+      const d = hypot(dx, dy);
       const step = p.speed * dt;
       if (d <= step || p.age >= p.maxAge) {
         p.x = p.tx; p.y = p.ty;