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/grid.js

@@ -54,6 +54,7 @@
 // off-centre unit may walk straight to a tile centre (ai.js planLeg, fear.js: else it steps back to its tile centre).
 
 import { ROWS, COLS } from './constants.js';
+import { hypot } from './detmath.js';
 
 /** Move cost of an obstacle-like tile (client `Tile.get_moveCost`: obstacle-like ? 1000 : 1). */
 export const OBSTACLE_COST = 1000;
@@ -376,7 +377,7 @@ export class Grid {
     for (let i = stack.length - 1; i >= 0; i--) {
       const a = stack[i], b = f.next[a];
       const ar = (a / COLS) | 0, ac = a - ar * COLS, br = (b / COLS) | 0, bc = b - br * COLS;
-      f.len[a] = f.len[b] + Math.hypot(br - ar, bc - ac);
+      f.len[a] = f.len[b] + hypot(br - ar, bc - ac);
     }
     return f.len[k];
   }
@@ -420,7 +421,7 @@ export class Grid {
   /** Length (tiles) of a polyline of [r,c] points. */
   static pathLength(pts) {
     let len = 0;
-    for (let i = 1; i < pts.length; i++) len += Math.hypot(pts[i][0] - pts[i - 1][0], pts[i][1] - pts[i - 1][1]);
+    for (let i = 1; i < pts.length; i++) len += hypot(pts[i][0] - pts[i - 1][0], pts[i][1] - pts[i - 1][1]);
     return len;
   }
 }