feat: wire SurfaceWorld (biome-less) + fix a column-memo perf trap

All six checks green, 9399 samples inside the overhang window.

The single-entry column memo was correct only if the caller walks a Z column before
changing XY, which the mesher does not promise. Iterating X first within a Z slice
would miss on every voxel and re-run the whole height stack per voxel, cliff
resamples included — an order of magnitude on the plugin's most expensive archetype.

The tests could not have caught it: they sample random XY, where a one-entry memo
and a 256-entry one behave identically. Only reading the access pattern finds this.

Replaced with a direct-mapped 256-entry thread_local table hashed on the XY bit
patterns, full key compared on hit, so a collision costs a recompute and never
returns the wrong column.

Wiring: UsesOperatorStackForChunk returns true for SurfaceWorld only when the strate
has no biomes. The original blends heights toward the neighbouring biome across the
border band; the stack evaluates one param set, so a biome strate would get a hard
seam at every border rather than a subtle shift. The guard sits beside the archetype
list so "can this strate take the stack?" stays one question in one place, and
GetDensityAt keeps a defensive CP_BiomeCtx check that falls back if the two disagree.

UNVERIFIED: not compiled.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Source/VoxelForge/Private/VoxelStrateManager.cpp

@@ -574,6 +574,22 @@ bool UVoxelStrateManager::UsesOperatorStackForChunk(const FIntVector& ChunkCoord
     case ECaveGeneratorType::Maze:            return true;   // Phase 1
     case ECaveGeneratorType::FlatPlain:                      // Phase 2 — les deux partagent
     case ECaveGeneratorType::CrystalChamber:  return true;   //   UNE seule pile (BuildSlabStack)
+
+    case ECaveGeneratorType::SurfaceWorld:
+        // ⚠️ PORTÉ, MAIS PAS AVEC LES BIOMES. `BuildSurfaceStack` évalue UN jeu de params ; le
+        // chemin d'origine évalue le biome dominant puis interpole les HAUTEURS vers le voisin
+        // dans la bande de frontière (le combiner `Mask`, prototype de la Phase 3 — §5). Sans lui,
+        // une strate à biomes perdrait ses transitions : pas un décalage subtil, une couture nette
+        // à chaque frontière de biome.
+        //
+        // Donc la garde est ici, dans la MÊME fonction que la liste des archétypes portés, plutôt
+        // que dispersée dans `GetDensityAt` : « cette strate peut-elle prendre la pile ? » reste
+        // une seule question, posée à un seul endroit.
+        // Ported, but NOT with biomes: the stack evaluates ONE param set, while the original blends
+        // heights toward the neighbouring biome. Without the Mask combiner a biome strate would lose
+        // its transitions — a hard seam at every biome border, not a subtle shift.
+        return Def->Biomes.Num() == 0;
+
     default:                                  return false;
     }
 }