perf(review): gate never deadlocks, collision radius, 20x faster digs

From the outside performance review (items 1-4; item 5 prototyped and reverted).

1. Collision gate: a support request outside the stamped desired set was cancelled by the rebuild
   it forced, and the gate held the pawn with gravity 0. Baseline deep-drop (floor at (0,0,-7),
   outside the aligned L0 ring): 1 hold of 30.0 s, never released. Fix: the target and 3 tiles
   below are retained through rebuilds at CollisionCritical; hold only when the swept capsule is
   about to enter pending support; release restores movement mode, gravity and downward speed.
   After: 3 holds, 2.21 s total, 112.8 m horizontal progress; gate off still penetrates (-7.4 cm).
2. Suspected cleanup regression (out-of-layout stacks): not real. 44 paired boundary tiles,
   30.9 ms vs 32.6 ms at b4ac996, hashes 44/44 identical. No change.
3. PawnCollisionRadiusChunks (new, default 1): L0 tiles outside the collision set stay visible
   without Chaos cooks; anchors and the gate's support column are unioned in. Cook requests
   ~217/min -> ~30/min (-86%). Critical support ready time did NOT improve (9.5 s -> 10.6 s in the
   deep-drop fixture; the wait is the cook itself, ~7.4 s cook-to-ready there).
4. Dig: exact pre-edit L0 density cached (one 35^3 grid, 171.5 KB), carve replays current mods on
   it. Sync remesh 99.4 ms -> 4.7 ms. 16/16 random carves hash-identical to full regeneration.
5. Parallel grid fill for the critical tile: 16/16 hash parity, but the waited-on tile was already
   meshed; the wait was collision cooking. Reverted.

Review G answered: M_Test1 still ray-marches VolTex0 (custom HLSL mini-sun shadow), so the density
volume is live. Known and left alone: outside the strate layout the fallback density can be solid
while the gate treats those tiles as air.
Tests: VoxelForgeCollisionDesiredTilesTest, VoxelForgePreEditDensityCacheTest; no-overlap, edit
ghost, descent corridor pass.

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

Source/VoxelForge/Private/VoxelMarchingCubesMesher.cpp

@@ -96,11 +96,14 @@ void UVoxelMarchingCubesMesher::InitializeConsoleSwitches()
 FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels, int32 Step, int32 InCellsPerAxis,
                                                        TArray<uint8>* OutCaptureGrid,
                                                        int32 BandZMinVox, int32 BandZMaxVox,
-                                                       int64* OutDensitySampleCount)
+                                                       int64* OutDensitySampleCount,
+                                                       FVoxelSharedDensityGrid* OutPreEditDensityGrid,
+                                                       const FVoxelSharedDensityGrid* InSharedDensityGrid)
 {
     TRACE_CPUPROFILER_EVENT_SCOPE(VoxelForge_MesherGenerateMesh);
     FVoxelMeshData MeshData;
     if (OutCaptureGrid) { OutCaptureGrid->Reset(); }
+    if (OutPreEditDensityGrid) { OutPreEditDensityGrid->Reset(); }
     if (OutDensitySampleCount) { *OutDensitySampleCount = 0; }
     if (!Generator) return MeshData;
     VoxelDensityProfile::FScopedTimer MesherProfileTimer(
@@ -383,7 +386,11 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
     // COUPE DE CONTENU PAR STRATE — restreint le maillage (et l'échantillonnage) aux cellules
     // dont l'intervalle Z chevauche la bande [BandZMinVox, BandZMaxVox] (voxels inclusifs).
     // Les tuiles à capture ne sont jamais bandées (niveau 0 — garde-fou ci-dessous).
-    if (OutCaptureGrid) { BandZMinVox = INT32_MIN; BandZMaxVox = INT32_MAX; }
+    if (OutCaptureGrid || OutPreEditDensityGrid)
+    {
+        BandZMinVox = INT32_MIN;
+        BandZMaxVox = INT32_MAX;
+    }
     int32 CzLo = 0, CzHi = CellsPerAxis - 1;
     if (BandZMinVox > INT32_MIN || BandZMaxVox < INT32_MAX)
     {
@@ -660,7 +667,8 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
     // capacité, donc plus de malloc/free de ~170 Ko (35³ floats) par tuile.
     static thread_local TArray<float> DensityGrid;
     DensityGrid.SetNumUninitialized(MDim * MDim * MDim);
-    const FVoxelSharedDensityGrid* ReuseGrid = SharedDensityGrid;
+    const FVoxelSharedDensityGrid* ReuseGrid = InSharedDensityGrid != nullptr
+        ? InSharedDensityGrid : SharedDensityGrid;
     const int32 SharedDeltaX = ReuseGrid
         ? (OriginVoxels.X - ReuseGrid->OriginVoxels.X) / Step : 0;
     const int32 SharedDeltaY = ReuseGrid
@@ -680,6 +688,17 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
         && SharedDeltaX + GridDim < ReuseGrid->Dim
         && SharedDeltaY + GridDim < ReuseGrid->Dim
         && SharedDeltaZ + GridDim < ReuseGrid->Dim;
+    const bool bCapturePreEditGrid = OutPreEditDensityGrid != nullptr
+        && !bUseSharedDensityGrid
+        && Step == 1 && CellsPerAxis == CHUNK_SIZE
+        && CzLo == 0 && CzHi == CellsPerAxis - 1;
+    if (bCapturePreEditGrid)
+    {
+        OutPreEditDensityGrid->OriginVoxels = OriginVoxels + FIntVector(-Step, -Step, -Step);
+        OutPreEditDensityGrid->Step = Step;
+        OutPreEditDensityGrid->Dim = MDim;
+        OutPreEditDensityGrid->Samples.SetNumUninitialized(MDim * MDim * MDim);
+    }
     // Bande de strate : seules les rangées Z réellement lues (cellules CzLo..CzHi + marges de
     // gradient) sont échantillonnées — le reste du tampon reste non initialisé et non lu.
     VoxelDensityProfile::FScopedTimer MesherDensityGridTimer(
@@ -731,10 +750,16 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
                     VoxelGenLOD::TileReachBlockIndex =
                         ((ReachBlockZ * ReachBlocksPerAxis) + ReachBlockY)
                             * ReachBlocksPerAxis + ReachBlockX;
+                    float PreEditDensity = 0.0f;
                     DensityGrid[LocalIndex] = Generator->GetDensityAt(
                         OriginVoxels.X + gx * Step,
                         OriginVoxels.Y + gy * Step,
-                        OriginVoxels.Z + gz * Step);
+                        OriginVoxels.Z + gz * Step,
+                        bCapturePreEditGrid ? &PreEditDensity : nullptr);
+                    if (bCapturePreEditGrid)
+                    {
+                        OutPreEditDensityGrid->Samples[LocalIndex] = PreEditDensity;
+                    }
                     if (OutDensitySampleCount)
                     {
                         ++(*OutDensitySampleCount);
@@ -848,6 +873,11 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
         }
     }
 
+    if (OutPreEditDensityGrid && !bCapturePreEditGrid)
+    {
+        OutPreEditDensityGrid->Reset();
+    }
+
     // Lecture grille (avec offset de marge) + gradient central depuis la grille.
     auto SampleG = [&](int32 gx, int32 gy, int32 gz) -> float
     {