wip(assembler): round S14 -- 4-way column cache + memoized MC gradients (output unchanged)

Worker-local XY column cache uses deterministic 4-way sets at LOD3-8: LOD4
misses 8,224 -> 1,437, conflicts 7,407 -> 0 (direct-map control: 6.62 s /
43.5 worker-s vs 4.24 s / 21.7). Exact LOD0 lattice gradients memoized per
tile: MC-exclusive 242 -> 201 ns/sample, two-pass loop unchanged. Hashes
identical, no recipe bump. Primary cold trace 4.24 s full set, ring 0.45 s,
75/75 at 40 m/s; a repeat ran uniformly ~40% slower on every LOD (machine
contention suspected); tick p95 to re-check on an idle machine. CVars
voxel.CoreColumnCacheDirectMapped / voxel.CoreMCGradientMemoization.
Report: Standalone/Research/WorldAssemblerM3/Round18.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

CODEMAP.md

@@ -1095,6 +1095,20 @@ generation output. Measurement-only stride and octave probes are confined to the
 audit and are not runtime controls. The result and limits are recorded in
 [`Standalone/Research/WorldAssemblerM3/Round17/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round17/REPORT.md).
 
+### 3.20 MC stage audit and coarse column-cache conflicts — World Assembler S14 / M3 Round 18
+
+`Standalone/Source/VoxelForge/Private/VoxelMarchingCubesMesher.cpp` records the sampled density
+grid, cell classification, grid-gradient normals, edge interpolation, and vertex/dedup stream
+stages for the BuildHost audit. Its exact per-worker gradient memo is controlled and trace-read
+back through `voxel.CoreMCGradientMemoization`. `Standalone/Core/src/WorldAssemblerInternal.h`
+keeps the 65,536-entry worker-local column cache, using the S13 direct-map for LOD0–2 and a
+deterministic four-way set-associative route for LOD3–8; `voxel.CoreColumnCacheDirectMapped=1`
+restores the S13 path for the negative control. Runtime CVar capture lives in
+`Source/VoxelForge/Private/VoxelWorld.cpp`; stage and conflict counts are reported by
+`Source/VoxelForgeEditor/Private/VoxelForgeExploreCommandlet.cpp`. The measurements, controls, and
+cold-path decision are in
+[`Standalone/Research/WorldAssemblerM3/Round18/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round18/REPORT.md).
+
 ## 4. The density pipeline (most-edited hot path)
 
 ### 4.1 `GetDensityWithParams` (TunnelNetwork) — VoxelGenerator.cpp:3749