wip(assembler): M1 round 6 -- owner walk fixes: 25 cm geometry scale, one height datum, carve/fill sign, faster relief meshing

Owner saw content floating above native terrain, spiky triangles, reversed carve/fill and late relief.
Causes: shared mesher placed Core tiles on a 12.5 cm grid; content/atmosphere asked strate ranges in
12.5 cm voxels; Core edits add density where the legacy diff subtracts. Horizon LOD8 bridge edge
5.657 -> 2.265 cells. Relief mesh p95 60.8 -> 42.0 ms (LOD0). New BuildHost checks for scale, height,
blueprint carve/fill and per-kind streaming lag.

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

Source/VoxelForge/Public/VoxelContentManager.h

@@ -59,6 +59,7 @@ class UStaticMesh;
 class UStaticMeshComponent;
 class UHierarchicalInstancedStaticMeshComponent;
 class UMaterialInterface;
+struct FCoreSurfaceSnapshot;
 
 // An active mini-sun light orb (a placed FStrateLandmark with bIsLightOrb). The terrain material marches
 // the density volume toward the nearest of these for raymarched shadows. Plain struct (not reflected);
@@ -97,6 +98,11 @@ public:
      *  plane). */
     void UpdateWater(const FVector& PlayerWorldPos);
 
+    /** Read immutable Core ground/water anchors for placement. / Lit les ancres natives sol/eau pour le placement Core. */
+    bool QueryNativeSurfaceAnchors(int32 StrateIndex, const FVector2D& LocalXYCm,
+                                   float& OutGroundLocalZCm, bool& bOutHasWater,
+                                   float& OutWaterLocalZCm) const;
+
     //--- DECORATIONS (distance-based world grid, async march) ----------------
     /** Stream decoration cells around the player: recompute the desired set on cell/strate change,
      *  launch async march tasks (capped), and apply finished results budgeted. Call every Tick. */
@@ -239,12 +245,15 @@ private:
     struct FDecoContext
     {
         const UVoxelStrateDefinition* Def = nullptr;   // game-thread only (biome/decoration resolve)
+        int32  StrateIndex    = INDEX_NONE;
         int32  RepChunkZ      = 0;          // representative chunk-Z for biome lookups
         float  TopVoxelZ      = 0.0f;       // strate band, voxel coords (march range)
         float  BottomVoxelZ   = 0.0f;
         float  WaterLocalZ    = -FLT_MAX;   // water surface, actor-local cm (-FLT_MAX = no water)
         bool   bHasWater      = false;
         bool   bSurfaceWorld  = false;      // heightfield archetype → use the GetSurfaceHeightAt oracle
+        bool   bNativeSurface = false;      // Core manifest is the height oracle and water datum / Le manifeste Core definit le relief et le niveau d'eau.
+        TSharedPtr<const FCoreSurfaceSnapshot, ESPMode::ThreadSafe> NativeSurface;
 
         // Strate biome field (PODs only → worker-safe). Empty ⇒ biomes disabled for this strate. The
         // worker resolves the dominant biome PER COLUMN (ResolveBiomeSampleAt) so decoration borders
@@ -337,6 +346,10 @@ private:
     UPROPERTY()
     UVoxelSettings* Settings = nullptr;
 
+    // Immutable Core manifest snapshot captured by decoration workers; workers never touch UObjects.
+    // Instantane immuable du manifeste Core pour les workers de decoration; aucun worker ne touche aux UObjects.
+    TSharedPtr<const FCoreSurfaceSnapshot, ESPMode::ThreadSafe> NativeSurfaceSnapshot;
+
     int32 Seed = 0;
 
     // Engine unit plane (/Engine/BasicShapes/Plane) reused for every water surface.
@@ -391,6 +404,6 @@ private:
 
     // Cached state so UpdateWater is a cheap no-op when neither the water level nor the snapped
     // player cell changed since last frame.
-    float     LastWaterZ    = -FLT_MAX;                          // strate water level (voxel Z)
+    float     LastWaterZ    = -FLT_MAX;                          // actor-local water height (cm) / Hauteur d'eau locale a l'acteur (cm).
     FIntPoint LastWaterCell = FIntPoint(INT32_MIN, INT32_MIN);   // snapped XY grid cell
 };