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/Public/VoxelWorld.h

@@ -152,6 +152,10 @@ struct FChunkResult
     // Exact pre-edit level-0 lattice for the bounded local-player dig cache. Sampled on the worker
     // with the tile's normal generator context and moved through the result queue.
     FVoxelSharedDensityGrid PreEditDensityGrid;
+    // Sparse points on the prior surface, used only by the explicit rendered-coverage audit.
+    // Captured from the mesher output and kept out of normal streaming results.
+    bool bCaptureSurfaceWitnesses = false;
+    TArray<FVector> SurfaceWitnesses;
 };
 
 /** One worker-produced mesh identity emitted only by the explicit parity harness. */
@@ -223,17 +227,16 @@ public:
     UPROPERTY()
     UVoxelAtmosphereManager* AtmosphereManager;
 
-    /** Player-centred density CLIPMAP streamed onto the GPU for the mini-sun raymarched shadow
-     *  system (forward rendering). Created in BeginPlay when Settings->bEnableDensityVolume is on.
-     *  Filled on worker threads (re-evaluating GetDensityAt), recentred toroidally as the player
-     *  moves, refilled locally on carve. See UVoxelDensityVolume. */
+    /** Optional player-centred density CLIPMAP for mini-sun raymarched shadows. It is absent when
+     *  Settings->bEnableDensityVolume is false, so no fill thread or texture uploads run. */
     UPROPERTY()
     UVoxelDensityVolume* DensityVolume;
 
     /** Shared Material Instance Dynamics that bind the density-volume textures + per-frame shadow params
      *  (clipmap transform + nearest orb) onto the terrain material(s). Keyed by BASE material so every
-     *  tile of a given base shares ONE MID (no batching cost). Created lazily in ApplyMeshToTile,
-     *  refreshed each Tick by UpdateTerrainMaterialParams. */
+     *  tile of a given base shares ONE MID (no batching cost). Created lazily in ApplyMeshToTile;
+     *  refreshed each Tick only while DensityVolume exists. When disabled, each MID receives neutral
+     *  VolP* values and null VolTex* bindings at creation. */
     UPROPERTY()
     TMap<TObjectPtr<UMaterialInterface>, TObjectPtr<UMaterialInstanceDynamic>> TerrainMIDs;
 
@@ -541,10 +544,15 @@ private:
     /** Apply the current TVP0..4 + level-0 volume texture to one MID (also used on MID creation). */
     void SetVolumeParamsOnMID(UMaterialInstanceDynamic* MID) const;
 
+    /** Put all terrain MIDs into the material's neutral, volume-disabled state. */
+    void SetDensityVolumeMaterialParamsDisabled();
+
     /** Explicitly opt-in headless streaming validation. This is a command-line-only harness path;
      * normal gameplay never moves the pawn here. It supplies a repeatable moving session and lets
      * -nullrhi request a graceful EndPlay so the clean latency summary is flushed. */
     void ConfigureHeadlessStreamingTest();
+    void AdvanceHeadlessDigMotionTest(FVector& InOutPlayerPosition, APawn* Pawn);
+    void RecordHeadlessDigMotionFrame(APawn* Pawn, const FVector& PlayerPosition, float DeltaTime);
     void AdvanceHeadlessStreamingTest(FVector& InOutPlayerPosition,
                                       FVector& InOutPlayerHeading, APawn* PlayerPawn);
     void AdvanceHeadlessCollisionGateStressTest(FVector& InOutPlayerPosition,