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/StreamingWorld.h

@@ -646,6 +646,16 @@ public:
     int32 GetWorkerCount()const{return WorkerCount;}
     uint64 GetCacheHits()const{return CacheHitCount.load(std::memory_order_relaxed);}
 
+    void SetMesherStageProfilingEnabled(bool Enabled)
+    {bProfileMesherStages=Enabled;}
+    FStandaloneCpuTile GenerateDiagnostic(const FVoxelTileKey& Key,
+        std::uint32_t GlobalOctaveDrop=0,std::uint32_t CellStride=1,
+        bool ForceGenerate=false)
+    {
+        FStandaloneTileJob Job;Job.Key=Key;Job.StartedAt=FClock::now();Job.QueuedAt=Job.StartedAt;
+        return Generate(Job,GlobalOctaveDrop,CellStride,ForceGenerate);
+    }
+
 private:
     FStandaloneStreamingUpdate UpdateDesiredSet(FStandaloneDesiredTileSet NewDesired,
         const std::vector<FVoxelTileKey>& ResidentKeys,uint64 CrossingIndex)
@@ -761,12 +771,14 @@ private:
         Wake.notify_all();
         return Stats;
     }
-    FStandaloneCpuTile Generate(const FStandaloneTileJob& Job)
+    FStandaloneCpuTile Generate(const FStandaloneTileJob& Job,
+        std::uint32_t DiagnosticGlobalOctaveDrop=0,std::uint32_t DiagnosticCellStride=1,
+        bool ForceGenerate=false)
     {
         const auto StartTime=Job.StartedAt;
         FStandaloneCpuTile Out;Out.Key=Job.Key;Out.RequestedCrossing=Job.RequestedCrossing;
         Out.QueuedAt=Job.QueuedAt;Out.StartedAt=Job.StartedAt;
-        if(MeshCache)
+        if(MeshCache&&!ForceGenerate)
         {
             if(const FStandaloneCpuTile* Cached=MeshCache->Find(Job.Key))
             {
@@ -805,6 +817,11 @@ private:
             Options.skipMaterialGroups=bSkipMaterialGroups;Options.skipUv=bSkipUV;Options.skipColors=bSkipColors;
             Options.captureColumnCoordinates=bCaptureColumnCoordinates;
             Options.lod8HorizonInteriorStepVoxels=Lod8HorizonInteriorStepVoxels;
+            Options.profileMesherStages=bProfileMesherStages&&!ForceGenerate;
+            Options.diagnosticGlobalOctaveDrop=DiagnosticGlobalOctaveDrop;
+            Options.diagnosticCellStride=std::max<std::uint32_t>(1,DiagnosticCellStride);
+            Options.diagnosticColumnInterpolationStride=
+                std::max<std::uint32_t>(1,DiagnosticCellStride);
             std::string Error;
             auto Result=CoreWorld->generateTile(
                 {{Job.Key.Coord.X,Job.Key.Coord.Y,Job.Key.Coord.Z},Job.Key.Level},Options,Error);
@@ -977,6 +994,7 @@ private:
     bool bSkipColors=false;
     bool bCaptureColumnCoordinates=false;
     int32 Lod8HorizonInteriorStepVoxels=0;
+    bool bProfileMesherStages=false;
     std::atomic<uint64> CacheHitCount{0};
     mutable std::mutex Mutex;
     std::condition_variable Wake;