wip(assembler): rounds L5+L6 -- Linux far-terrain captures and screen-space gate (world unchanged)

Linux offscreen capture (EGL/GLES pbuffer; llvmpipe in the sandbox) and an opt-in screen-space gate
for LOD>=3 (normal distance projected at each LOD's closest display distance, 1 px at 1440p =
0.61 mrad). The gate rejects the S13 octave-drop and stride-2 controls, fault injections, and a
vertical-vs-screen control. Finding: today's far meshes differ from the field by up to 92 px, and
LOD swaps snap mesa rims by tens of px (p50 0 px), a silhouette effect of plain LOD switching. The
octave cutoff was dropped: nothing qualified, and octaves are a small share of far time. Off-path
cost: reviewer interleaved W12 within noise vs L4 (+0.6% median). Hashes = L4.
Largest far lever measured: column work outside noise, 34% of far worker time (duplicate XY across tiles).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ED33VnqepD4YY6gDAAA832

Standalone/Source/VoxelForge/Private/VoxelMarchingCubesMesher.cpp

@@ -1768,6 +1768,7 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
     const int32 GroundSurfaceTriangleCount=GroundTris.Num()/3;
     const int32 CapSurfaceTriangleCount=CapTris.Num()/3;
     TArray<uint8> GroundSkirtFaceMasks,CapSkirtFaceMasks;
+    const uint64 SkirtStageStartCycles=OutStageTimings?FPlatformTime::Cycles64():0;
     if (bGenerateSkirts && (GroundTris.Num() + CapTris.Num()) > 0)
     {
         const float ExtentCm = (float)(CellsPerAxis * Step) * GeometryVoxelSize;
@@ -2007,6 +2008,10 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
             EmitSkirts(GroundTris,GroundSkirtFaceMasks);
             EmitSkirts(CapTris,CapSkirtFaceMasks);
         }
+    if(OutStageTimings)
+        OutStageTimings->SkirtMilliseconds=static_cast<double>(
+            FPlatformTime::Cycles64()-SkirtStageStartCycles)
+                *FPlatformTime::GetSecondsPerCycle64()*1000.0;
 
     // F17 — concatène sol PUIS sky-cap : un run d'indices contigu par polygroup (voir
     // FVoxelMeshData::NumCeilingTriangles). Mêmes triangles qu'avant, seul l'ordre change.
@@ -2059,6 +2064,8 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
             AuditInterpolationCycles,AuditInterpolationSamples,SurfaceCellCalls);
         OutStageTimings->VertexDedupStreamMilliseconds=ScaleSampledCycles(
             AuditDedupStreamCycles,AuditDedupStreamSamples,SurfaceCellCalls);
+        OutStageTimings->SkirtMilliseconds=std::max(0.0,
+            OutStageTimings->SkirtMilliseconds);
         const double TotalMilliseconds=static_cast<double>(
             FPlatformTime::Cycles64()-StageMesherStartCycles)*CycleToMs;
         OutStageTimings->OtherMilliseconds=FMath::Max(0.0,TotalMilliseconds
@@ -2066,7 +2073,8 @@ FVoxelMeshData UVoxelMarchingCubesMesher::GenerateMesh(FIntVector OriginVoxels,
             -OutStageTimings->CellClassificationMilliseconds
             -OutStageTimings->GradientNormalsMilliseconds
             -OutStageTimings->EdgeInterpolationMilliseconds
-            -OutStageTimings->VertexDedupStreamMilliseconds);
+            -OutStageTimings->VertexDedupStreamMilliseconds
+            -OutStageTimings->SkirtMilliseconds);
     }
 
     return MeshData;