wip(assembler): round L7 -- shared column cache across workers: whole view 16-19% faster, world unchanged

SharedColumnCache.h: a world-owned, bounded 4-way second-tier cache of exact column values
(heightfield and floating islands), keyed by recipe/plan identity plus content hash, XY and island
id. Lock-free seqlock reads; set-local spin lock on misses. The FLazy thread-local tier stays
first. 83.6% of repeated column work was cross-worker; formula evaluations -60.9%.
W12 L1 view: Luna 2.165 -> 1.754 s; reviewer interleaved 2.059 -> 1.722 s. 8 m crossing new tiles
82 -> 52 ms. +66 MiB peak RSS. All hashes (whole view, per-LOD, exact queries, islands seed 0)
identical at W1/W8/W12, Clang off and GCC default. MSVC/UBT not compiled here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CSFYgsdW8ejZSyiMf4qRSX

Standalone/Source/StreamingWorld.h

@@ -428,12 +428,14 @@ public:
                                            bool bInSkipColors=false,
                                            VoxelForgeCore::World* InCoreWorld=nullptr,
                                            bool bInCaptureColumnCoordinates=false,
-                                           int32 InLod8HorizonInteriorStepVoxels=0)
+                                           int32 InLod8HorizonInteriorStepVoxels=0,
+                                           bool bInSharedColumnCache=true)
         :World(&InWorld),WorkerCount(FMath::Clamp(InWorkerCount,1,16)),bHashDensity(bInHashDensity),
          MeshCache(InMeshCache),MaxLevelFilter(InMaxLevelFilter),
          CoreWorld(InCoreWorld),bSkipUvAndColors(bInSkipUvAndColors),bSkipMaterialGroups(bInSkipMaterialGroups),
          bSkipUV(bInSkipUV),bSkipColors(bInSkipColors),
          bCaptureColumnCoordinates(bInCaptureColumnCoordinates),
+         bSharedColumnCache(bInSharedColumnCache),
          Lod8HorizonInteriorStepVoxels(std::max(0,InLod8HorizonInteriorStepVoxels))
     {
         VoxelGenLOD::InitializeConsoleSwitches();
@@ -816,6 +818,7 @@ private:
             Options.skipUvAndColors=bSkipUvAndColors;
             Options.skipMaterialGroups=bSkipMaterialGroups;Options.skipUv=bSkipUV;Options.skipColors=bSkipColors;
             Options.captureColumnCoordinates=bCaptureColumnCoordinates;
+            Options.useSharedColumnCache=bSharedColumnCache;
             Options.lod8HorizonInteriorStepVoxels=Lod8HorizonInteriorStepVoxels;
             Options.profileMesherStages=bProfileMesherStages&&!ForceGenerate;
             Options.diagnosticGlobalOctaveDrop=DiagnosticGlobalOctaveDrop;
@@ -993,6 +996,7 @@ private:
     bool bSkipUV=false;
     bool bSkipColors=false;
     bool bCaptureColumnCoordinates=false;
+    bool bSharedColumnCache=true;
     int32 Lod8HorizonInteriorStepVoxels=0;
     bool bProfileMesherStages=false;
     std::atomic<uint64> CacheHitCount{0};