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/VoxelGenerator.cpp

@@ -578,6 +578,26 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
                     VoxelDensityOps::BuildSlabStack(CP_OpStack, CP_Slab, Seed,
                                                     OriginSpineRadius, StrateManager);
                     break;
+
+                case ECaveGeneratorType::SurfaceWorld:
+                    // Même garde dégénérée que les autres, et une SECONDE garde : la pile ne sait
+                    // pas encore mélanger les biomes. `UsesOperatorStackForChunk` refuse déjà les
+                    // strates à biomes, mais un `CP_BiomeCtx` valide ici voudrait dire que les deux
+                    // sources d'information se contredisent — auquel cas on retombe sur le `switch`,
+                    // parce qu'un monde non porté est récupérable et un monde faux ne l'est pas.
+                    // A valid biome context here would mean the two sources of truth disagree; fall
+                    // back rather than generate a world with seams at every biome border.
+                    if (CP_Surface.StrateTopWorldZ - CP_Surface.StrateBottomWorldZ <= 0.0f
+                        || CP_BiomeCtx.IsValid())
+                    {
+                        CP_UseOpStack = false;
+                        break;
+                    }
+                    OpCtx.StrateTopWorldZ    = CP_Surface.StrateTopWorldZ;
+                    OpCtx.StrateBottomWorldZ = CP_Surface.StrateBottomWorldZ;
+                    VoxelDensityOps::BuildSurfaceStack(CP_OpStack, CP_Surface, Seed,
+                                                       OriginSpineRadius, StrateManager);
+                    break;
                 default:
                     // UsesOperatorStackForChunk ne rend true que pour les archétypes portés, donc
                     // on ne devrait jamais arriver ici. Si ça arrive, retomber sur le `switch`