wip(assembler): round S1 -- one Core voxel-size source; per-tile feature lists (islands)

AVoxelWorld::CoreVoxelSizeCm (default 25, 32 voxels/tile) flows through plan, field,
mesher, content, water, atmosphere, edits and collision. Staged-owner BuildHost passes at
20 cm (SurfaceWorld + islands, W1/W8/W16, anchors, BP edits, collision) with a fixed-25cm
negative control; default-size hashes match round 4; Core off stays legacy 12.5 cm.
Reusable BuildWorldAssemblyFeatureTileListV1; wired into FloatingIslands only so far.
256-island stress: tile p95 112.5 -> 2.0 ms, 0 mismatches, dropped-feature control fails.
Content placement measured (no populated cells): island ground search 7.25 ms/cell vs
legacy 0.29 ms -- open. Outside-review evaluation in Round5/REPORT.md.

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

Source/VoxelForge/Public/VoxelContentManager.h

@@ -120,6 +120,32 @@ public:
      *  launch async march tasks (capped), and apply finished results budgeted. Call every Tick. */
     void UpdateDecorations(const FVector& PlayerWorldPos);
 
+    /** Headless audit of worker-cell and steady UpdateDecorations cost using the active world route.
+     *  Audit sans rendu du coût par cellule worker et de UpdateDecorations avec la route active. */
+    struct FDecorationPlacementCostAudit
+    {
+        bool bNativeSurface = false;
+        bool bNativeFloatingIslands = false;
+        bool bSyntheticWorkload = false;
+        int32 NearEntries = 0;
+        int32 FarEntries = 0;
+        int32 SampledCells = 0;
+        int32 ColumnsPerCell = 0;
+        int32 Placements = 0;
+        int32 AnchorChecks = 0;
+        double MaximumAnchorErrorCm = 0.0;
+        double MaximumDensityResidual = 0.0;
+        double DensityQueriesPerCell = 0.0;
+        double SurfaceQueriesPerCell = 0.0;
+        double CellMedianMilliseconds = 0.0;
+        double CellP95Milliseconds = 0.0;
+        double UpdateMedianMilliseconds = 0.0;
+        double UpdateP95Milliseconds = 0.0;
+    };
+    bool MeasurePlacementCostForValidation(const FVector& PlayerWorldPos,
+        int32 SampleCellCount, int32 FrameSampleCount,
+        FDecorationPlacementCostAudit& OutAudit, FString& OutError);
+
     //--- LANDMARKS (rare large objects on a coarse hash lattice — the "mini-suns") -----------
     /** Stream rare landmark objects around the player. Unlike decorations, these sit on a COARSE hash
      *  lattice (cell = `FStrateLandmark::SpacingChunks` chunks), so cost scales with the number of
@@ -176,6 +202,12 @@ public:
         bool       bInstanced = false;
         FTransform Xf = FTransform::Identity;
     };
+    struct FPlacementBuildCounters
+    {
+        int32 Columns = 0;
+        int32 DensityQueries = 0;
+        int32 SurfaceQueries = 0;
+    };
     /** A finished cell march: a snapshot of the resolved decoration list + the spawn commands. */
     struct FDecoCellResult
     {
@@ -259,6 +291,8 @@ private:
         const UVoxelStrateDefinition* Def = nullptr;   // game-thread only (biome/decoration resolve)
         int32  StrateIndex    = INDEX_NONE;
         int32  RepChunkZ      = 0;          // representative chunk-Z for biome lookups
+        float  VoxelSizeCm    = VOXEL_SIZE; // active field voxel size: Core when enabled, legacy otherwise
+        float  ActiveToOwnerVoxelScale = 1.0f; // map active Core units to the legacy generator's voxel units
         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)
@@ -303,7 +337,8 @@ private:
                                 const TArray<FStrateDecoration>& Entries,
                                 const TArray<int32>& EntryBiome, uint32 InSeed,
                                 int32 Spacing, float Step, int32 MaxCrossings, float ColumnDepth,
-                                TArray<FDecoSpawn>& OutSpawns);
+                                TArray<FDecoSpawn>& OutSpawns,
+                                FPlacementBuildCounters* OutCounters = nullptr);
 
     // Each step operates on ONE grid (G = NearGrid or FarGrid). LaunchDecoTasks throttles against the
     // COMBINED in-flight count (OtherInFlight = the other grid's in-flight cells) so the two grids share