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/world.js

@@ -12,7 +12,7 @@ function drawWorld() {
   R.rain = (R.rain || 0) * 0.98 + rainAt(Math.max(0, Math.min(W - 1, Math.floor(R.camX + bw / 2)))) * 0.02;
   const rk = R.rain;
   // Smooth look: the colours below are still worked out per grain, but each grain also gets a
-  // class (alpha byte) and the GPU draws rounded shapes from them (see smooth.js)
+  // class (alpha byte) and the GPU draws rounded shapes from them (see smooth-shaders.js)
   const sm = R.smoothOn = SET.look === 'smooth' && !R.lens && typeof smoothAvailable === 'function' && smoothAvailable();
   const bpx = R.backPx, tpx = R.treePx, fpx = R.fieldPx, mmask = R.moldMask;
   if (sm) {