Refactor: merge duplicated rules, split the largest files by topic (t21)

- One helper each for wall and mold refunds, tube edges and lanes, pipe text, piece dimensions,
  font lookup, reach circles and surface averaging.
- previews, smooth, notebook, materials, worldgen and selection split into topic files
  (preview-drill, preview-selection, smooth-shaders, notebook-market, notebook-blueprints,
  material-tables, worldgen-helpers, selection-pickers, selection-panels).
- Every suite's output identical apart from timings; seeds make the same worlds.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/sim/world.js

@@ -67,8 +67,9 @@ function depthU(y) {
 }
 // the y at a fraction u of the underground depth (the inverse of depthU)
 function depthY(u) { return Math.round(surfBase + (!shallowD ? u * underD : u <= 0.3 ? u / 0.3 * shallowD : shallowD + (u - 0.3) / 0.7 * (underD - shallowD))); }
+function meanSurface() { let s = 0; for (let x = 0; x < W; x++) s += surfY[x]; return s / W; }
 function computeAmbient() {
-  let sm = 0; for (let x = 0; x < W; x++) sm += surfY[x]; sm /= W;
+  const sm = meanSurface();
   for (let y = 0; y < H; y++) {
     let a = 18;
     if (y > sm) a += Math.min(60, (y - sm) * 0.04);