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.

eslint.config.js

@@ -39,6 +39,10 @@ const rules = {
   'preserve-caught-error': 'warn',
 };
 
+// ECMA-262 §21.3.2: the Math functions whose results are "implementation-approximated".
+const APPROXIMATED_MATH = ['acos', 'acosh', 'asin', 'asinh', 'atan', 'atanh', 'atan2', 'cbrt', 'cos', 'cosh', 'exp',
+  'expm1', 'hypot', 'log', 'log1p', 'log10', 'log2', 'pow', 'sin', 'sinh', 'tan', 'tanh'];
+
 const nodeFiles = [
   'server/**/*.js',
   'shared/**/*.js',
@@ -79,6 +83,20 @@ export default [
     },
     rules,
   },
+  {
+    // The sim runs in every player's browser and on the server, and must give the same bits everywhere. ECMA-262
+    // leaves these Math functions (and **) implementation-approximated: engines differ in the last bits.
+    files: ['server/sim/**/*.js'],
+    rules: {
+      'no-restricted-properties': ['error', ...APPROXIMATED_MATH.map((property) => ({
+        object: 'Math', property, message: 'implementation-approximated (engines differ in the last bits): use server/sim/detmath.js',
+      }))],
+      'no-restricted-syntax': ['error',
+        { selector: "BinaryExpression[operator='**']", message: 'implementation-approximated: use powi from server/sim/detmath.js' },
+        { selector: "AssignmentExpression[operator='**=']", message: 'implementation-approximated: use powi from server/sim/detmath.js' },
+      ],
+    },
+  },
   {
     // Node test runner, plus browser tests whose page.evaluate callbacks use DOM globals.
     // PIXI is the page global those render tests read inside evaluate().