measure(step2a): candidate bins alone are slower -- the per-sample GetDensityAt call must go

Editor-only ablation (explore commandlet, -regioneval, default off): 8^3 bins for rooms,
room-floor joins and tunnel chains, but every point still goes through GetDensityAt.
259^3 ring, seed 1337, six workers:
  live  : 6.776 s wall, 29.16 s worker CPU, 1.679 us/sample, 21.2 candidate visits/sample
  binned: 19.54 s wall, 50.02 s worker CPU, 2.879 us/sample, 16.4 candidate visits/sample
Correctness: zero Q16 (x16, ties away from zero) disagreements on the full ring and ~284k probes
on 4 more seeds (composer recipes, warped rooms, passages, landings, strate edges, edits);
shrinking room reach to 0.5 -> 18,107 Q16 / 735 sign differences (the check bites).
Not the planned region program: the step-2 question is still open. Baseline for the next attempt:
1.68 us CPU per sample on the live path.

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

CODEMAP.md

@@ -117,9 +117,10 @@ remains `0`, and lateral regions remain gated off.
 |------|------|
 | `../../VoxelForge.uplugin` | Plugin manifest. Runtime module `VoxelForge` plus the Editor-only `VoxelForgeEditor` commandlet module. Beta. |
 | `VoxelForge.Build.cs` | Deps: Core, CoreUObject, Engine, **GameplayTags**, **RealtimeMeshComponent**. |
-| `Tools/VoxelForgeTest.ps1` | The single guarded test entry point. `-Scenario canonical|owner|probe|perf|tests|surface-fall|crossing|gate-stress|visual-edit|visual-lod|visual-gap-walk|visual-gap-inject|dig-motion|parity|horizon|horizon-render|determinism`, optional `-Build`, `-Assets owner|default`, `-Cvars`, absolute `-Out`, `-Label`, `-HorizonLevel`, `-HorizonRenderDistanceChunks`, `-ParityExportSize=32|64|128`, `-ParityExportTile=x,y,z`, and `-SharedDDC`; stages the host/assets, isolates user/DDC/config state, serializes one Unreal launch at a time, enforces the 8 GiB/30-minute guards, and emits compact `result.json`/`summary.txt` plus full logs. Parity can render its game leg with `-EnableRender` and compares the selected export region against the exact streamed tile hashes. `visual-gap-walk` records camera-frustum surface-point coverage at 6000 cm/s; `visual-gap-inject` proves an early component hide is detected; `dig-motion` logs per-frame vertical velocity, gate, support readiness, and movement mode. `-Cvars @{ 'voxel.TestGapAuditLegacyHandoff'=1 }` replays the old visibility handoff for A/B audit baselines. Horizon traces report desired/loaded tiles, resident stream geometry, worker seconds, steady wall time, and peak private memory; `horizon-render` captures the owner view; `determinism` compares per-tile hashes across the default CRT path and `-voxel.CrtFma3=0`. |
+| `Tools/VoxelForgeTest.ps1` | The single guarded test entry point. `-Scenario canonical|owner|probe|perf|tests|surface-fall|crossing|gate-stress|visual-edit|visual-lod|visual-gap-walk|visual-gap-inject|dig-motion|parity|horizon|horizon-render|determinism|region-eval`, optional `-Build`, `-Assets owner|default`, `-Cvars`, absolute `-Out`, `-Label`, `-HorizonLevel`, `-HorizonRenderDistanceChunks`, `-ParityExportSize=32|64|128`, `-ParityExportTile=x,y,z`, and `-SharedDDC`; stages the host/assets, isolates user/DDC/config state, serializes one Unreal launch at a time, enforces the 8 GiB/30-minute guards, waits for 6 GiB free RAM before launches, and emits compact `result.json`/`summary.txt` plus full logs. Parity can render its game leg with `-EnableRender` and compares the selected export region against the exact streamed tile hashes. `visual-gap-walk` records camera-frustum surface-point coverage at 6000 cm/s; `visual-gap-inject` proves an early component hide is detected; `dig-motion` logs per-frame vertical velocity, gate, support readiness, and movement mode. `-Cvars @{ 'voxel.TestGapAuditLegacyHandoff'=1 }` replays the old visibility handoff for A/B audit baselines. Horizon traces report desired/loaded tiles, resident stream geometry, worker seconds, steady wall time, and peak private memory; `horizon-render` captures the owner view; `determinism` compares per-tile hashes across the default CRT path and `-voxel.CrtFma3=0`; `region-eval` runs the isolated step-2 lattice comparison and candidate-bin ablation.
 | `Public/VoxelForgeModule.h` / `Private/VoxelForgeModule.cpp` | `FVoxelForgeModule` boilerplate (Startup/Shutdown just log). |
-| `Source/VoxelForgeEditor/VoxelForgeEditor.Build.cs` / `Private/VoxelForgeExploreCommandlet.*` | Editor/commandlet-only `-run=VoxelForgeExplore`; owns bounded render/`VF_` walk/OBJ export orchestration and is not linked by Game/Shipping. Optional `-exporttilex/y/z` sets an exact L0 tile origin for controlled export/game parity; omitting all three preserves the existing center-derived origin. |
+| `Source/VoxelForgeEditor/VoxelForgeEditor.Build.cs` / `Private/VoxelForgeExploreCommandlet.*` | Editor/commandlet-only `-run=VoxelForgeExplore`; owns bounded render/`VF_` walk/OBJ export orchestration and is not linked by Game/Shipping. Optional `-exporttilex/y/z` sets an exact L0 tile origin for controlled export/game parity; omitting all three preserves the existing center-derived origin. `-regioneval=1` runs the default-off step-2 research harness against live `GetDensityAt`. |
+| `Source/VoxelForgeEditor/Private/VoxelForgeRegionEvalExperiment.*` | Six-partition lattice driver for the commandlet-only step-2 experiment: cold 259³ timing, bitwise repeat hashes, fixed 16-bit quantized comparisons, and exact candidate-visit attribution. It calls the live final-field API in both modes; the prepared mode selects editor-only 8³ SDF candidate bins. |
 | `Public/VoxelGeometryHash.h` | Header-only geometry digest shared by commandlet export and runtime tile-hash dump; hashes mesh counts and the vertex/normal/UV/color/triangle arrays with the historical CRC order. |
 | `Public/VoxelStats.h` / `Private/VoxelStats.cpp` | `stat VoxelForge` DWORD counters for tile classification, skipping, meshing, operator-stack verdicts, and cave-bail diagnosis. Each `Cave Bail *` counter names one guard; `Cave Bail No Stack` counts slots with no buildable stack (no definition, or a degenerate strate). |
 
@@ -458,6 +459,13 @@ Header is rich with inline docs. Two namespaces + a per-chunk cache system.
   — deterministic hashing for room/tunnel placement (no storage, infinite worlds).
 - Cache structs (h:520-950): `FCachedRoom`, `FCachedTunnel`, `FCachedPit`,
   `FCachedChimney`, `FCachedColumn`, `FChunkSDFCache`.
+- `FChunkSDFBrickIndex` is an editor-only-at-use 8-voxel 3D bin beside the existing XY index. The
+  default-off `SetResearchBrickBinningEnabled` selector builds/selects it only for the explore
+  commandlet. `VF_BuildResearchBrickIndices` uses the cached room cull reach (shape, floor relief,
+  blend and terrain-op envelope), room-floor-join bounds, and the union of SDF/world tunnel
+  centerline influence and bounding-sphere bounds. Source item indices stay in ascending order;
+  invalid or oversized grids fall back to today's XY broad phase. This step-2 candidate-bin
+  ablation still routes every lattice point through the canonical `GetDensityAt` final field.
 - `namespace VoxelCaveMorphology`:
   | Function | .cpp line | Role |
   |----------|-----------|------|