fix: signed-overflow UB in VoxelHash made cave placement compiler-dependent; all generators LOD0-8; speed matrix (L60)

VoxelHash::Cell/Pair, legacy Cellular3D and the shared Core cellular hash added 0x7FFFFFFF to a signed
int32 before casting to uint32; overflow is undefined, so Clang and GCC placed cave passage 1 differently
(first differing sample at LOD1 tile (0,64,-512)). The bias is now done in unsigned arithmetic, in both the
standalone copy and the Unreal plugin header (Source/VoxelForge/Public/VoxelCaveMorphology.h). Native
caves, motifs and lattice now stream LOD0-8 with skirts and cross-level transitions. New verify.py
--speed-matrix runs every generator on one asserted-identical view (5 interleaved runs): motifs 1.33,
profiles 1.30-1.32, caves 1.41, natural SW 1.67, lattice 1.70, FI 4.93 s whole view. fast/medium/full PASS,
Clang/GCC peer hashes equal. Research R29 (open-edge ledge) and R30 (compiler-divergence suspects).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Core/src/VoxelForgeFieldShared.h

@@ -394,9 +394,9 @@ VF_FIELD_INLINE float VF_FieldOwnerCellular(float x,float y,float z)
     {
         const VF_FieldI32 nx=cellX+dx,ny=cellY+dy,nz=cellZ+dz;
         const VF_FieldU32 h=VF_FieldVoxelHashMix(
-            VF_FIELD_CAST_U32(nx+0x7FFFFFFF)
-            ^VF_FieldVoxelHashMix(VF_FIELD_CAST_U32(ny+0x7FFFFFFF)*2654435761u)
-            ^VF_FieldVoxelHashMix(VF_FIELD_CAST_U32(nz+0x7FFFFFFF)*374761393u));
+            (VF_FIELD_CAST_U32(nx)+0x7FFFFFFFu)
+            ^VF_FieldVoxelHashMix((VF_FIELD_CAST_U32(ny)+0x7FFFFFFFu)*2654435761u)
+            ^VF_FieldVoxelHashMix((VF_FIELD_CAST_U32(nz)+0x7FFFFFFFu)*374761393u));
         const float fpx=VF_FIELD_CAST_F32(dx)
             +VF_FIELD_CAST_F32(h&0xFFFFFFu)*(1.0f/16777216.0f)-fracX;
         const VF_FieldU32 hy=VF_FieldVoxelHashMix(h^0x12345678u);