passages: memoized exact MC midpoints + wider conservative sweep bounds (L101, exact vs L100)

Off-grid census by caller (MC midpoints, skirts, far shells); all 63.0M sweep misses accounted
(1.9M exact-route fallbacks, 61.1M counted bound skips). Exact MC midpoint queries memoized by
float bits (Unreal enables it too); feature bounds expanded. L100/L101 whole-view hashes equal in
all four strata (v216 unchanged); 0 added/removed crossings; 15 capture pairs pixel-identical.
Near-ready @W12 L100 -> L101: FI 1.77 -> 1.65, TN 1.66 -> 1.54, SW 0.85 -> 0.84, VS 1.03 -> 1.03 s
(L98 2.28/2.23/1.02/1.11). Fast tier PASS; Linux UE host builds. Unreal arrival traces not obtained
(the runner wrongly applied the 6 GiB address-space cap to Unreal).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Source/VoxelForge/Private/VoxelWorld.cpp

@@ -14128,6 +14128,7 @@ void AVoxelWorld::LoadCoreTile(const FVoxelTileKey& Tile, EVoxelTileWorkPriority
         Options.hashDensity = bCaptureHashes;
         Options.legacyDirectMappedColumnCache=bLegacyDirectMappedColumnCache;
         Options.memoizeMcGradients=bMemoizeCoreMCGradients;
+        Options.exactMidpointDensityMemo=true;
         if(Tile.Level==0)
         {
             const int32 ActiveStreamingTasks=GetActiveGenerationTaskCount();