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/render/setup.js

@@ -129,7 +129,7 @@ const BG = {
 let SKY_R, SKY_G, SKY_B;
 function buildSky() {
   SKY_R = new Uint8Array(H); SKY_G = new Uint8Array(H); SKY_B = new Uint8Array(H);
-  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++) {
     const f = Math.min(1, y / (sm + 20));
     SKY_R[y] = 88 + f * 92; SKY_G[y] = 140 + f * 70; SKY_B[y] = 198 + f * 34;
@@ -194,3 +194,7 @@ function screenToWorld(sx, sy) {
   return { x: R.camX + sx * dpr / R.zp, y: R.camY + sy * dpr / R.zp };
 }
 function worldToScreenPx(x, y) { return { x: (x - R.camX) * R.zp, y: (y - R.camY) * R.zp }; }
+
+function renderMonoFont() {
+  if (!R.monoFont) R.monoFont = getComputedStyle(document.documentElement).getPropertyValue('--font-mono').trim() || 'monospace';
+}