wip(assembler): M2 round 3 -- opt-in hybrid sharp mesher (WorldAssemblerHybridV1) in the Unreal Core route

Default MarchingCubesV1 unchanged (W1/W8/W16 = round-6 hashes). Hybrid DC only in LOD0 interior cells
with tagged features; tile-border cells and LOD1+ stay MC. BuildHost: fixtures repeat exactly, cooked
collision bounds = render bounds, same-LOD borders pass, sampled SurfaceWorld cliff bit-identical to MC.
Open: tagged tiles cost 5.7-6.7x MC; corner-top field contact error 20 cm; 5 MC doorway LOD cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Core/src/VoxelForgeCore.cpp

@@ -890,11 +890,10 @@ namespace VoxelForgeCore
             if(const auto found=impl_->tileRegions.find(key);found!=impl_->tileRegions.end())
                 page=found->second;
         }
-        const FStandaloneShapeWorld& tileShape=page?*page:*impl_->shape;
+        const FStandaloneShapeWorld* tileShapeContext=page?page.get():impl_->shape.get();
         UVoxelMarchingCubesMesher mesher;
         mesher.SetGenerator(world.Generator.get());
         mesher.SetGeometryVoxelSizeCm(25.0f);
-        mesher.SetStandaloneShapeWorld(&tileShape);
         mesher.bStandaloneSkipUvAndColors=options.skipUvAndColors;
         mesher.bStandaloneSkipUV=options.skipUv;mesher.bStandaloneSkipColors=options.skipColors;
         mesher.bStandaloneSkipMaterialGroups=options.skipMaterialGroups;
@@ -904,6 +903,54 @@ namespace VoxelForgeCore
         const int32 level=key.lod,extent=CHUNK_SIZE<<level;
         const FVoxelTileKey nativeKey=ToNative(key);
         const FIntVector origin=nativeKey.OriginVoxels();
+        if(options.mesherRecipe!=MesherRecipe::marchingCubesV1
+            &&options.mesherRecipe!=MesherRecipe::worldAssemblerHybridV1)
+        {error="tile mesher recipe version is unsupported";return std::nullopt;}
+        TArray<FVoxelHybridSharpFeatureBox> sharpFeatures;
+        if(options.mesherRecipe==MesherRecipe::worldAssemblerHybridV1&&level==0&&impl_->worldAssembly)
+        {
+            constexpr std::int64_t SubvoxelsPerVoxel=WorldAssemblySubvoxelsPerVoxel;
+            const std::array<std::int64_t,3> tileMin={
+                static_cast<std::int64_t>(origin.X)*SubvoxelsPerVoxel,
+                static_cast<std::int64_t>(origin.Y)*SubvoxelsPerVoxel,
+                static_cast<std::int64_t>(origin.Z)*SubvoxelsPerVoxel};
+            const std::array<std::int64_t,3> tileMax={
+                static_cast<std::int64_t>(origin.X+extent)*SubvoxelsPerVoxel,
+                static_cast<std::int64_t>(origin.Y+extent)*SubvoxelsPerVoxel,
+                static_cast<std::int64_t>(origin.Z+extent)*SubvoxelsPerVoxel};
+            for(const auto& stratum:impl_->worldAssembly->plan.strates)
+            for(const auto& feature:stratum.features)
+            {
+                // Type 2 is the plan's finite, hard box-solid exception; the type-4 shelf is
+                // smooth-min blended and therefore remains on MC until it carries sharp planes.
+                // Le type 2 est une boite solide nette; l'etagere type 4 reste MC car elle est lisse.
+                if(feature.type!=2)continue;
+                bool overlaps=true;
+                for(std::size_t axis=0;axis<3;++axis)
+                    overlaps=overlaps&&feature.minimumSubvoxels[axis]<=tileMax[axis]
+                        &&feature.maximumSubvoxels[axis]>=tileMin[axis];
+                if(!overlaps)continue;
+                FVoxelHybridSharpFeatureBox box;
+                box.StableId=feature.stableId;
+                for(std::size_t axis=0;axis<3;++axis)
+                {
+                    box.MinimumSubvoxels[axis]=feature.minimumSubvoxels[axis];
+                    box.MaximumSubvoxels[axis]=feature.maximumSubvoxels[axis];
+                }
+                sharpFeatures.Add(box);
+            }
+            if(!sharpFeatures.IsEmpty())
+            {
+                // Streamed analytic pages omit the native assembly plan. Use the immutable base
+                // shape only for tagged tiles so their mesher samples the same field as the tags.
+                // Les pages analytiques omettent le plan natif ; utiliser la base uniquement pour
+                // les tuiles balisees afin d'echantillonner le champ correspondant aux tags.
+                tileShapeContext=impl_->shape.get();
+                mesher.SetHybridSharpFeatures(MoveTemp(sharpFeatures));
+            }
+        }
+        const FStandaloneShapeWorld& tileShape=*tileShapeContext;
+        mesher.SetStandaloneShapeWorld(&tileShape);
         int32 cells=level<world.FullResClipLevels?CHUNK_SIZE:FMath::Max(1,world.CoarseTileCells);
         int32 step=FMath::Max(1,extent/cells);
         bool bFeatureRefinedResolution=false;