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/StreamBenchmarkHeadless.cpp

@@ -37,6 +37,7 @@ namespace
         bool ProfileColumnOverlap=false;
         bool ForceGenerate=false;
         bool MeshCoverage=false;
+        bool ScreenSpaceGate=false;
         bool HasSeedOverride=false;
         int SeedOverride=0;
         bool HasRenderDistanceOverride=false;
@@ -48,6 +49,9 @@ namespace
         bool HasCamera=false;
         bool AutoSurfaceCamera=false;
         FVector CameraMeters=FVector::ZeroVector;
+        float CameraYawDegrees=0.0f;
+        float CameraPitchDegrees=0.0f;
+        std::filesystem::path MeshExportPath;
     };
 
     bool ParseInteger(std::string_view text,int& output)
@@ -61,13 +65,15 @@ namespace
         return true;
     }
 
-    bool ParseCamera(std::string_view text,FVector& meters)
+    bool ParseCamera(std::string_view text,FVector& meters,float& yawDegrees,float& pitchDegrees)
     {
         const std::string values(text);
         float x=0.0f,y=0.0f,z=0.0f,yaw=0.0f,pitch=0.0f;
         if(std::sscanf(values.c_str(),"%f,%f,%f,%f,%f",&x,&y,&z,&yaw,&pitch)!=5)
             return false;
         meters={x,y,z};
+        yawDegrees=yaw;
+        pitchDegrees=pitch;
         return true;
     }
 
@@ -85,10 +91,12 @@ namespace
                 std::printf("Usage: %s [owner_world.json] --world-assembler-prototype --stream-benchmark --stream-pipeline [options]\n"
                     "  --workers=N --clip-radius=N --max-clip-level=N --max-level=N\n"
                     "  --camera=X,Y,Z,YAW,PITCH (position in metres) --stream-hash\n"
+                    "  --mesh-export=FILE (write the exact generated tile meshes for offline rendering)\n"
                     "  --profile-column-overlap --far-horizon-lod8-step=N --force-generate\n"
                     "  --seed=N --recipe=surface-world|floating-islands\n"
                     "  --auto-surface-camera (place the 27-tile cube on this seed's surface)\n"
                     "  --mesh-coverage (report non-empty tiles and indexed triangles)\n"
+                    "  --screen-space-gate (measure display error and historical negative controls)\n"
                     "  --render-distance-chunks=N (0 disables the extra far ring)\n"
                     "  --tls-probe=N (Linux worker TLS/stack measurement; N=0..12)\n"
                     "  --tls-probe-touch=none|surface-world|floating-islands (first-use probe)\n"
@@ -103,6 +111,7 @@ namespace
             else if(arg=="--profile-column-overlap")options.ProfileColumnOverlap=true;
             else if(arg=="--force-generate")options.ForceGenerate=true;
             else if(arg=="--mesh-coverage")options.MeshCoverage=true;
+            else if(arg=="--screen-space-gate")options.ScreenSpaceGate=true;
             else if(arg=="--auto-surface-camera")options.AutoSurfaceCamera=true;
             else if(arg.starts_with("--recipe="))
             {
@@ -135,10 +144,17 @@ namespace
             }
             else if(arg.starts_with("--camera="))
             {
-                if(!ParseCamera(arg.substr(9),options.CameraMeters))
+                if(!ParseCamera(arg.substr(9),options.CameraMeters,
+                    options.CameraYawDegrees,options.CameraPitchDegrees))
                 {std::fprintf(stderr,"Invalid --camera value: %.*s\n",static_cast<int>(arg.size()),arg.data());return false;}
                 options.HasCamera=true;
             }
+            else if(arg.starts_with("--mesh-export="))
+            {
+                options.MeshExportPath=std::filesystem::path(arg.substr(14));
+                if(options.MeshExportPath.empty())
+                {std::fprintf(stderr,"--mesh-export needs a file path\n");return false;}
+            }
             else if(arg.starts_with("--workers="))
             {if(!readInt("--workers=",options.WorkerCount))return false;}
             else if(arg.starts_with("--clip-radius="))
@@ -297,5 +313,6 @@ int main(int argc,char** argv)
         cameraVoxels,options.WorkerCount,options.MaxLevel,VoxelForgeCore::DetailVariant::coherent,
         scheduler,options.StreamHash,false,false,false,false,coldStart,
         options.ProfileColumnOverlap,options.FarHorizonLOD8InteriorStepVoxels,
-        options.MeshCoverage);
+        options.MeshCoverage,options.MeshExportPath,options.CameraYawDegrees,
+        options.CameraPitchDegrees,options.ScreenSpaceGate);
 }