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/Core/src/WorldAssemblerInternal.h

@@ -1031,6 +1031,7 @@ namespace world_assembly_detail
     // Audit-only override; normal tile generation resets this to zero before evaluating a recipe.
     // Surcharge reservee aux mesures; la generation remet zero avant d'evaluer une recette.
     inline thread_local std::uint32_t HeightfieldColumnOctaveDrop = 0;
+    inline thread_local std::int64_t HeightfieldColumnInterpolationStrideSubvoxels = 0;
 
     inline std::int32_t FractalNoiseQ16(std::int64_t xQ16, std::int64_t yQ16,
         std::int64_t zQ16, std::uint32_t octaves, bool ridged = false)
@@ -1506,6 +1507,43 @@ namespace world_assembly_detail
     inline HeightfieldPointSample CachedColumn(const WorldAssemblyRecipeV1& recipe,
         const WorldAssemblyStratumPlanV1& plan,std::int64_t x,std::int64_t y)
     {
+        const std::int64_t InterpolationStride=HeightfieldColumnInterpolationStrideSubvoxels;
+        if(InterpolationStride>0)
+        {
+            const std::int64_t X0=FloorDiv(x,InterpolationStride)*InterpolationStride;
+            const std::int64_t Y0=FloorDiv(y,InterpolationStride)*InterpolationStride;
+            const std::int64_t X1=X0+InterpolationStride,Y1=Y0+InterpolationStride;
+            const std::int64_t Fx=x-X0,Fy=y-Y0;
+            struct FScopedInterpolationSampling
+            {
+                std::int64_t Previous=HeightfieldColumnInterpolationStrideSubvoxels;
+                ~FScopedInterpolationSampling()
+                {HeightfieldColumnInterpolationStrideSubvoxels=Previous;}
+            } SamplingScope;
+            HeightfieldColumnInterpolationStrideSubvoxels=0;
+            const HeightfieldPointSample C00=CachedColumn(recipe,plan,X0,Y0);
+            const HeightfieldPointSample C10=CachedColumn(recipe,plan,X1,Y0);
+            const HeightfieldPointSample C01=CachedColumn(recipe,plan,X0,Y1);
+            const HeightfieldPointSample C11=CachedColumn(recipe,plan,X1,Y1);
+            const auto Lerp=[&](std::int64_t A,std::int64_t B,std::int64_t T)
+            {return A+((B-A)*T)/InterpolationStride;};
+            const auto Bilinear=[&](std::int64_t A,std::int64_t B,
+                std::int64_t C,std::int64_t D)
+            {
+                const std::int64_t Top=Lerp(A,B,Fx),Bottom=Lerp(C,D,Fx);
+                return Lerp(Top,Bottom,Fy);
+            };
+            HeightfieldPointSample Out=C00;
+            Out.groundHeightSubvoxels=Bilinear(C00.groundHeightSubvoxels,
+                C10.groundHeightSubvoxels,C01.groundHeightSubvoxels,
+                C11.groundHeightSubvoxels);
+            Out.capHeightSubvoxels=Bilinear(C00.capHeightSubvoxels,
+                C10.capHeightSubvoxels,C01.capHeightSubvoxels,
+                C11.capHeightSubvoxels);
+            return Out;
+        }
+        if(HeightfieldColumnOctaveDrop!=0)
+            return EvaluateColumn(recipe,plan,x,y);
         auto& cache=HeightfieldColumnCache.Get();
         auto& nextWay=HeightfieldColumnCacheNextWay.Get();
         HeightfieldColumnCacheEntry* replacement=nullptr;
@@ -2058,6 +2096,19 @@ void Heightfield2D::resetThreadColumnCacheStats(bool captureCoordinates, bool cl
 void Heightfield2D::setThreadColumnCacheLegacyDirectMapped(bool enabled) noexcept
 { world_assembly_detail::HeightfieldColumnCacheLegacyDirectMapped=enabled; }
 
+void Heightfield2D::setThreadColumnOctaveDrop(std::uint32_t octaveDrop) noexcept
+{ world_assembly_detail::HeightfieldColumnOctaveDrop=octaveDrop; }
+
+std::uint32_t Heightfield2D::threadColumnOctaveDrop() noexcept
+{ return world_assembly_detail::HeightfieldColumnOctaveDrop; }
+
+void Heightfield2D::setThreadColumnInterpolationStrideSubvoxels(std::int64_t stride) noexcept
+{ world_assembly_detail::HeightfieldColumnInterpolationStrideSubvoxels=std::max<std::int64_t>(0,stride); }
+
+std::int64_t Heightfield2D::threadColumnInterpolationStrideSubvoxels() noexcept
+{ return world_assembly_detail::HeightfieldColumnInterpolationStrideSubvoxels; }
+
+
 const HeightfieldColumnCacheStats& Heightfield2D::threadColumnCacheStats() noexcept
 { return world_assembly_detail::HeightfieldColumnStats; }