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/blocks.js

@@ -71,6 +71,15 @@ function blockIntegrity(b) {
   void v;
   return n;
 }
+// One cell of a wall or mold being taken back: metal is counted toward bars (returns 1), other
+// walls give their rubble back.
+function removeWallCell(i, wall) {
+  const m = mat[i];
+  if (BAR_OF[m] && PH[wall] === PH_ITEM) { setCell(i, EMPTY, temp[i], 0); return 1; }
+  if (YLD[m] && bagAdd(YLD[m], 1, false) === 0) { setCell(i, YLD[m], temp[i], 0); droppedLast++; } // bag full: it crumbles in place
+  else setCell(i, EMPTY, temp[i], 0);
+  return 0;
+}
 function removeBlock(bx, by) {
   const b = by * BW + bx, code = blockP[b], v = blockV[b];
   if (!code) return 0;
@@ -82,9 +91,7 @@ function removeBlock(bx, by) {
     const m = mat[i], p = part[i];
     if (code === BP_WALL) {
       if (PH[m] === PH_SOLID && !(flags[i] & F_NAT) && !PARTMAT[m] && m !== SPRING) {
-        if (BAR_OF[m] && PH[v] === PH_ITEM) { metal++; setCell(i, EMPTY, temp[i], 0); }
-        else if (YLD[m] && bagAdd(YLD[m], 1, false) === 0) { setCell(i, YLD[m], temp[i], 0); droppedLast++; } // bag full: it crumbles in place
-        else setCell(i, EMPTY, temp[i], 0);
+        metal += removeWallCell(i, v);
         n++;
       }
     } else if (isPipeCode(code)) {