fix(streaming): no false gate on digs, no see-through at tile handoffs; density volume off

- Dig near the pawn: a same-tile collision recook withdrew readiness although the old body was
  still installed, so the gate held a standing pawn (957 of 1,931 frames over 8 near digs).
  A tile now stays ready during a recook while collision is still wanted and its existing body is
  valid. After: 0 gate frames, 0 falling frames over 2,604 frames. Dig directly under the pawn
  still falls (~127 cm, max ~502 cm/s). The reported "accelerating toward the ground" was not
  reproduced; the false hold is the measured part.
- See-through: the retire logic only waited for replacements that overlapped the old tile in
  volume and missed desired neighbours touching at a face/edge/corner. Handoff now includes
  contact. New surface-witness gap audit (24 surface points per tile, skirts excluded, catches an
  injected early hide). 6000 cm/s walk, 75 s: 3 gaps / 51 frames / longest 603 ms -> 0.
  Dig session: 0 -> 0. Overlap audit and edit ghost stay at 0.
- bEnableDensityVolume defaults to false; when off: no volume, no fill thread, uploads or tick
  work, no volume-dependent proof bypass; terrain MIDs get VolP5.z = 0 (enable off), other VolP*
  black, VolTex* null, so M_Test1's volume nodes return their neutral result.

Gate: deep gate-on passes (79.9 m, 3 holds, -0.101 cm), gate-off penetrates (-5.1 cm).

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

Source/VoxelForge/Private/VoxelMarchingCubesMesher.cpp

@@ -1065,6 +1065,7 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
     // Émis en DOUBLE FACE (deux orientations) pour s'afficher quel que soit le côté caméra / le
     // matériau. Une arête de surface posée sur une face de frontière a sa coordonnée d'axe EXACTE
     // (l'interpolation MC garde fixe l'axe de la face) → comparaison flottante exacte fiable.
+    MeshData.NumSurfaceVertices = MeshData.Vertices.Num();
     if (bGenerateSkirts && (GroundTris.Num() + CapTris.Num()) > 0)
     {
         const float ExtentCm = (float)(CellsPerAxis * Step) * VOXEL_SIZE;