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/VoxelSettings.h

@@ -305,7 +305,7 @@ public:
 
 	// Master switch. OFF = no volume built, no fill tasks, no GPU cost (terrain unlit by orbs).
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Lighting")
-	bool bEnableDensityVolume = true;
+	bool bEnableDensityVolume = false;
 
 	// Per-axis resolution of EACH clip level (cells). Memory per level ≈ Res³ bytes (R8). 128 →
 	// ~2 MB/level; 192 → ~7 MB; 256 → ~16 MB. Higher = crisper near shadows + bigger startup fill.