wip(assembler): round L3 -- Core forces FP contraction off and refuses to generate if it can't

CoreNamespacePreamble.h applies per-compiler contraction-off pragmas (Clang, MSVC, GCC; unknown
compiler = #error) and a fused-multiply-add self-test that World::compile runs before building.
Clang -ffp-contract=on and GCC's default (fast) now reproduce the L1/Windows hashes (they diverged
in L2); Clang -ffp-contract=fast ignores the pragma and is refused at startup (negative control).
Seed sweep with every cube on real terrain: 48 seeds x 2 recipes x 3 builds x W1/W12, 576 runs,
0 mismatches, 0 empty meshes. Cost +0.5% (2.326 -> 2.337 s, interleaved). World unchanged.
MSVC/UBT not compiled here: Windows to-do list in Linux3/REPORT.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Standalone/Source/StreamBenchmarkHeadless.cpp

@@ -7,12 +7,15 @@
 
 #include <algorithm>
 #include <chrono>
+#include <cmath>
+#include <cstdint>
 #include <cstdio>
 #include <cstdlib>
 #include <filesystem>
 #include <fstream>
 #include <iostream>
 #include <iterator>
+#include <limits>
 #include <memory>
 #include <string>
 #include <string_view>
@@ -33,6 +36,7 @@ namespace
         bool StreamHash=false;
         bool ProfileColumnOverlap=false;
         bool ForceGenerate=false;
+        bool MeshCoverage=false;
         bool HasSeedOverride=false;
         int SeedOverride=0;
         bool HasRenderDistanceOverride=false;
@@ -41,6 +45,7 @@ namespace
         bool TlsProbeRequested=false;
         int TlsProbeWorkers=0;
         bool HasCamera=false;
+        bool AutoSurfaceCamera=false;
         FVector CameraMeters=FVector::ZeroVector;
     };
 
@@ -81,6 +86,8 @@ namespace
                     "  --camera=X,Y,Z,YAW,PITCH (position in metres) --stream-hash\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"
                     "  --render-distance-chunks=N (0 disables the extra far ring)\n"
                     "  --tls-probe=N (Linux worker TLS/stack measurement; N=0..12)\n"
                     "  --process-affinity=MASK --process-priority=Normal --frame-limit=FPS (viewer compatibility only)\n",
@@ -93,6 +100,8 @@ namespace
             else if(arg=="--stream-hash")options.StreamHash=true;
             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=="--auto-surface-camera")options.AutoSurfaceCamera=true;
             else if(arg.starts_with("--recipe="))
             {
                 const std::string_view recipe=arg.substr(std::string_view("--recipe=").size());
@@ -221,6 +230,48 @@ int main(int argc,char** argv)
         &&!StandaloneWorldAssemblyPrototype::AttachFloatingIslandWorldAssembly(
             *ownerWorld,ownerJsonRoot,worldAssembly,error))
     {std::fprintf(stderr,"Floating-island assembly setup failed: %s\n",error.c_str());return 2;}
+    if(options.AutoSurfaceCamera)
+    {
+        constexpr std::int64_t UnitsPerVoxel=VoxelForgeCore::WorldAssemblySubvoxelsPerVoxel;
+        if(options.FloatingIslandsRecipe)
+        {
+            const VoxelForgeCore::FloatingIslandAnchorV1* chosen=nullptr;
+            for(const auto& islandSet:worldAssembly.floatingIslands)
+                for(const auto& island:islandSet.manifest.plan.islands)
+                    if(!chosen||island.radiusSubvoxels>chosen->radiusSubvoxels)chosen=&island;
+            if(!chosen)
+            {std::fprintf(stderr,"[SeedSurfacePlacement] FAIL: island plan has no anchors\n");return 2;}
+            cameraVoxels={
+                static_cast<float>(chosen->centerSubvoxels[0])/UnitsPerVoxel,
+                static_cast<float>(chosen->centerSubvoxels[1])/UnitsPerVoxel,
+                static_cast<float>(chosen->centerSubvoxels[2])/UnitsPerVoxel};
+            std::printf("[SeedSurfacePlacement] recipe=floating-islands seed=%d island_radius_vox=%.3f camera_voxel=(%.6f,%.6f,%.6f) status=PASS\n",
+                seed,static_cast<double>(chosen->radiusSubvoxels)/UnitsPerVoxel,
+                cameraVoxels.X,cameraVoxels.Y,cameraVoxels.Z);
+        }
+        else
+        {
+            const auto xSubvoxels=static_cast<std::int64_t>(std::llround(cameraVoxels.X*UnitsPerVoxel));
+            const auto ySubvoxels=static_cast<std::int64_t>(std::llround(cameraVoxels.Y*UnitsPerVoxel));
+            const auto currentZSubvoxels=static_cast<std::int64_t>(std::llround(cameraVoxels.Z*UnitsPerVoxel));
+            std::int64_t selectedGround=0,bestDistance=std::numeric_limits<std::int64_t>::max();
+            for(const auto& stratum:worldAssembly.plan.strates)
+            {
+                VoxelForgeCore::Heightfield2D field(worldAssembly.recipe,stratum);
+                if(!field.isValid())continue;
+                const auto column=field.point(xSubvoxels,ySubvoxels,0);
+                const std::int64_t distance=std::abs(column.groundHeightSubvoxels-currentZSubvoxels);
+                if(distance<bestDistance)
+                {bestDistance=distance;selectedGround=column.groundHeightSubvoxels;}
+            }
+            if(bestDistance==std::numeric_limits<std::int64_t>::max())
+            {std::fprintf(stderr,"[SeedSurfacePlacement] FAIL: no valid SurfaceWorld heightfield\n");return 2;}
+            cameraVoxels.Z=static_cast<float>(static_cast<double>(selectedGround)/UnitsPerVoxel);
+            std::printf("[SeedSurfacePlacement] recipe=surface-world seed=%d ground_voxel=%.6f camera_voxel=(%.6f,%.6f,%.6f) status=PASS\n",
+                seed,static_cast<double>(selectedGround)/UnitsPerVoxel,
+                cameraVoxels.X,cameraVoxels.Y,cameraVoxels.Z);
+        }
+    }
     std::printf("[WorldAssemblerPlanTiming] total_ms=%.3f status=PASS\n",
         std::chrono::duration<double,std::milli>(Clock::now()-planStart).count());
 
@@ -233,5 +284,6 @@ int main(int argc,char** argv)
     return StandaloneStreamBenchmark::RunPipelinedStreamingBenchmark(ownerWorld,snapshot,
         cameraVoxels,options.WorkerCount,options.MaxLevel,VoxelForgeCore::DetailVariant::coherent,
         scheduler,options.StreamHash,false,false,false,false,coldStart,
-        options.ProfileColumnOverlap,options.FarHorizonLOD8InteriorStepVoxels);
+        options.ProfileColumnOverlap,options.FarHorizonLOD8InteriorStepVoxels,
+        options.MeshCoverage);
 }