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/VoxelDensityOpStack.cpp

@@ -449,13 +449,31 @@ namespace
 
         const FColumn& GetColumn(float WorldX, float WorldY) const
         {
-            thread_local FColumn C{};
-            thread_local uint64  CachedId = 0;
-            thread_local float   CachedX = FLT_MAX, CachedY = FLT_MAX;
+            // ⚠️ POURQUOI UNE TABLE ET PAS UNE SEULE ENTRÉE. Un mémo à une entrée n'est correct que
+            // si l'appelant descend une colonne Z avant de changer de XY. Le mesher n'en promet
+            // RIEN — s'il itère X en premier dans une tranche Z, chaque voxel raterait et on
+            // relancerait toute la pile de hauteur par voxel, cliff compris (4 resamples
+            // structurels). Ce n'est pas « un peu plus lent », c'est un ordre de grandeur sur
+            // l'archétype le plus cher du plugin.
+            //
+            // Table à correspondance directe, 256 entrées, clé COMPLÈTE comparée sur touche : une
+            // collision ne peut que coûter un recalcul, jamais rendre une mauvaise colonne.
+            //
+            // A single-entry memo is only correct-by-luck: it assumes the caller walks a Z column
+            // before changing XY, which the mesher does not promise. Direct-mapped 256-entry table
+            // with the FULL key compared on hit — a collision costs a recompute, never a wrong column.
+            struct FSlot { uint64 Id; float X, Y; FColumn C; };
+            thread_local FSlot Slots[256] = {};
 
-            if (CachedId != InstanceId || CachedX != WorldX || CachedY != WorldY)
+            const uint32 HX = *reinterpret_cast<const uint32*>(&WorldX);
+            const uint32 HY = *reinterpret_cast<const uint32*>(&WorldY);
+            const uint32 Idx = (HX * 0x9E3779B9u ^ HY * 0x85EBCA6Bu) >> 24;   // [0,255]
+
+            FSlot& S = Slots[Idx];
+            if (S.Id != InstanceId || S.X != WorldX || S.Y != WorldY)
             {
-                CachedId = InstanceId;  CachedX = WorldX;  CachedY = WorldY;
+                S.Id = InstanceId;  S.X = WorldX;  S.Y = WorldY;
+                FColumn& C = S.C;
 
                 C.TerrainZ = TerrainStack.EvalHeight(WorldX, WorldY);
                 C.CeilSurf = CeilingStack.EvalHeight(WorldX, WorldY);
@@ -482,7 +500,7 @@ namespace
                     if (Slope > KINDA_SMALL_NUMBER) { C.DirX = GX / Slope; C.DirY = GY / Slope; }
                 }
             }
-            return C;
+            return S.C;
         }
 
         /** Le champ structurel nu — l'overhang s'en sert pour emprunter la roche amont. */