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>

docs/agent-log.md

@@ -4,6 +4,20 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-24: t21 phase 4: duplicates merged, largest files split (Claude Code, Codex)
+
+- Merged seven real duplicates: wall refunds (`removeWallCell`, walls and molds), tube edges and
+  lanes (`entTubeEdge`, `entTubeLane`), pipe text, piece dimensions, font lookup, reach circles,
+  surface averaging (`meanSurface`). Look-alikes left alone: data tables, sprite rows, GLSL, and
+  the hot movement and heat code.
+- Split by topic: `ui/previews.js` -> `preview-drill.js`, `preview-selection.js`;
+  `render/smooth.js` -> `smooth-shaders.js`; `ui/notebook.js` -> `notebook-market.js`,
+  `notebook-blueprints.js`; `sim/materials.js` -> `material-tables.js`; `sim/worldgen.js` ->
+  `worldgen-helpers.js`; `ui/selection.js` -> `selection-pickers.js`, `selection-panels.js`.
+  `render/world.js` left whole (one large function).
+- Every suite's output identical apart from timings; seeds generate the same worlds.
+  `run_all.py --browser` ALL PASSED.
+
 ## 2026-10-24: t21 phase 3: speed-ups that change nothing (Claude Code, Codex)
 
 - Profiled a Huge world (6144x2048) headless and in the browser. After a load every area is