docs: the operator stack is the only density path

- CODEMAP: data-flow diagram, §3.2b/§3.2d, generator and strate-manager
  rows, test rows and §4.2/§5 now describe the stack-only path; the
  deleted archetype functions, bUseOperatorStack and the Not Op Stack
  stats are gone; line hints re-synced for VoxelGenerator.cpp,
  VoxelStrateManager.cpp and VoxelStrateDefinition.h.
- ARCHITECTURE §8.1: archetype table lists stack builders; §8.10 surface
  column and shaft cache entries name their current owners.
- REVIEW_FINDINGS: tick "Operator stack as the only density path"; add
  the no-strate-manager fallback world as its own open item.
- fable-idea: T1.a status names the current column caches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01URDGLsu7c4nunqNFPJEFPU

fable-idea.md

@@ -32,7 +32,8 @@ So: normals can cost as much as the entire density grid; a SurfaceWorld chunk do
 
 ### Tier 1 — high win / low risk / small-medium effort (do these first)
 
-> **STATUS 2026-07-05: Tier 1 COMPLETE.** T1.a ✅ (surface-column cache, now file-scope `GSurfColCache`);
+> **STATUS 2026-07-05: Tier 1 COMPLETE.** T1.a ✅ (surface-column cache; since cleanup/2026-09 the density
+> path's copy is the surface stack's `FSurfaceColumnSource` memo and `GSurfColCache` serves `ClassifyTile` only);
 > T1.b ✅ (grid-based normals inline in `GenerateMesh`);
 > T1.c ✅ (`bShouldCreateCollision` = level 0 only, VoxelWorld.cpp ApplyMeshToTile); T1.d ✅ (v2
 > `ClassifyTile`, trace-verified −44 % worker CPU); T1.e ✅ (`MaxMeshAppliesPerFrame`, default 4);
@@ -160,7 +161,7 @@ Mobs need two things the world didn't give them: (1) to *exist* away from the lo
   mandatory. AI is server-authoritative (§9.6). ★ Build when NPCs actually land.
 
 **F20 — Biome-selected surface terrain ops (terrace / cliff / layer-lines / overhang / spike / hole). SPEC 2026-07-07.**
-Terrain ops are cave-only today (per-room in `GetDensityWithParams`; `GetSurfaceDensity` applies NONE — that's
+Terrain ops were cave-only at the time (per-room in `GetDensityWithParams`; the SurfaceWorld density applied NONE — that's
 the "ops don't work on the surface" report). This brings them to the SURFACE, as a **biome** property,
 **conditioned on local terrain** so they read geological instead of random. (Slots in ahead of the later
 cave-system redo, which will add biome support cave-side reusing this same op→biome model.)