research(step2d): a simpler field is fast (81 ns/sample) -- but it is not the owner's world; NOT a pass

Editor-only, default off (voxel.ResearchRedefinedDensityField). The redefined field is a seeded,
periodic cross-corridor pattern per strate plus the authored passages, landings and a shaft tree,
with four value-noise detail bands at every near-surface 25 cm sample and sound 8^3 brick proofs.
259^3 ring, six workers: 308 ms median (467 ms without skips) vs 8.9 s for the live field.
Detail, boundary-artefact, determinism (1 vs 6 workers, two runs) and bound-audit checks pass.

Why it is not a pass: the slices show a mostly-solid field with a few straight corridors, where the
owner's world is mostly open space; rooms, the tunnel network, composer recipes and archetypes are
gone. Much of the speed comes from 60% of bricks being proven solid in that simpler world.
What it does show: detail at full resolution plus brick skipping costs ~80-130 ns/sample; the
cost of the live field is how rooms/tunnels/passages are represented, not the detail or the lattice.

The two walkability tests were redirected to the research field in the working tree; that change
is NOT committed (saved as scratchpad step2d-test-redirect.patch) so they keep testing the live world.

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

Saved/Research/step2d/step2d_report.md

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+# Rebuild step 2d research report
+
+Date: 2026-09-30  
+Branch: `rebuild/step2d-redefined`  
+Scope: `Plugins/VoxelForge`
+
+## Result
+
+The editor-only research field passes the 259³, 25 cm ring target on six workers. Its final
+median is **308.177 ms** across three runs (295.325–317.807 ms), below both the 600 ms step-2
+bar and the 350 ms step-3 target. The old field measured 8,900.965 ms median on the same ring.
+Preparation, including the 135.528 ms world setup, is reported separately and excluded from
+the ring timings.
+
+The six-worker driver claims contiguous batches of 16 bricks dynamically. This balances dense
+surface work against uniform interiors without changing evaluation order within a brick or any
+sample value.
+
+The old and new density fields intentionally differ. The new evaluator remains disabled by
+default; the live field is still selected unless an editor research test explicitly enables
+`voxel.ResearchRedefinedDensityField`.
+
+## Field definition
+
+Each strate gets a deterministic, seed-offset periodic cross-corridor signed field. Rounded
+landing chambers, approaches, and a deterministic landing-to-hub bridge join those passages.
+Authored passage carving and floor geometry remain active. The VerticalShafts strate adds a
+seeded shaft tree and a continuous 48-voxel cavern horizon to connect its authored mouths.
+
+Four continuous value-noise bands use periods of 8, 4, 2, and 1 lattice samples, with total
+absolute amplitude bounded by 5.25 density units. No coarse density field is interpolated onto
+the lattice: every sample in a brick that may contain the zero set evaluates the full field and
+all four detail bands. Research mode also declines the live tile classifier's early-out so its
+mesh cannot be culled using bounds for the old field.
+
+## Timing
+
+All rows use 17,373,979 samples, a 259³ window at origin `(-129,-129,-481)`, stride 1, spacing
+25 cm, and six observed workers unless noted.
+
+| Field and path | Wall time | Worker CPU | CPU/sample | Bricks skipped | Samples skipped | Geometry evaluations/sample |
+|---|---:|---:|---:|---:|---:|---:|
+| Existing live field, six-worker median (range 8,890.120–8,923.020 ms) | 8,900.965 ms | 37.70 s | 2,170.09 ns | 0% | 0% | 6.734 |
+| Redefined field, all points evaluated, six workers | 467.212 ms | 2.313 s | 133.10 ns | 0% | 0% | 1.000 |
+| Redefined field, sound 8³ proofs, six-worker median (range 295.325–317.807 ms) | **308.177 ms** | 1.41 s | 80.94 ns | 60.54% | 63.06% | 0.371 |
+
+The optimized pass skipped 23,794 of 39,304 bricks and wrote uniform values to 10,956,496
+proven samples. It evaluated full density at the remaining 6,417,483 samples. The 0.371 new
+geometry figure counts a full density evaluation or brick macro proof against every ring point;
+the old path performed about 6.734 room/tunnel/passage primitive evaluations per point. Each
+evaluated new-field point computes four noise bands; averaged across the whole ring, including
+proved skips, this is 1.477 noise-band evaluations per sample.
+
+The final run's accumulated worker-phase timers were 1.79 s for full density evaluation, 0.04 s
+for uniform fills, and 0.01 s for brick proofs. Those phase timers sum work across workers, so
+they are not disjoint wall-time slices. The longest worker took 307.76 ms; measured parallel
+overhead after that was 0.42 ms. The remaining wall time is in full-detail evaluation of the
+6.42 million nonuniform samples.
+
+## Detail, bounds, and boundaries
+
+The roughness metric is six-neighbor Laplacian RMS over samples within 12 density units of the
+surface, measured on the shared ring. The new field has fewer surface-band samples because its
+topology is different (3,116,658 versus 15,515,231); on those bands its lag-1 RMS is 8.978 versus
+6.248 for the old field, and lag-2 RMS is 9.169 versus 8.646. A deliberately smoothed new-field
+control (detail scale 0) falls to 1.315 and 1.933. In the same selected 64³ mesh region, triangle
+counts were 180 old, 5,067 new, and 1,902 smoothed. The metric and mesh both detected the
+smoothing control. The measured detail batch also matched direct evaluation exactly at 8,488
+samples.
+
+For the actual scale-1 field, 186 correctly skipped bricks were audited across 165,390 final
+field samples including the marching-cubes halo; there were zero sign violations. The bound
+includes the full 5.25 detail supremum, the macro Lipschitz bound, and a floating-point margin;
+edits, passage bounds, and chunk/strate boundaries decline the proof. A negative control that
+classified with detail scale 0 and checked the supported scale-4 field found 35,847 wrong-sign
+samples across 26,572 claimed bricks, so the audit detects an undersized detail bound.
+
+Near-surface gradient-jump RMS was 3.420 at 8³ brick boundaries versus 8.694 in brick interiors,
+3.423 at chunk boundaries, and 3.225 at 128-sample macro-cell boundaries versus 10.036 in those
+cell interiors. These checks found no boundary spike consistent with stair-stepping. The field
+is analytic and periodic across those boundaries; the brick classifier only replaces proven
+uniform interiors.
+
+## Determinism and functional checks
+
+The six-worker output matched on two independent runs and matched the one-worker output:
+
+- Quantized hash: `7dec3f2d65781237`
+- Raw-float hash: `b50465c1eaa85921`
+
+The required `StrateConnectivity` and `StrateConnectivityRefinement` automation tests passed in
+`step2d_functional_22`. The fitted VerticalShafts audit reported 16/16 routes directly confirmed
+at step 2, zero endpoint snaps, and no exact step-1 fallback. Its 16-seed shaft-tree audit had
+0/10,372 orphan paths and passed its order-independence check. The bounded retry setting in this
+test is 128 so the richer full-detail field can reject more coarse false edges before accepting
+a path.
+
+`PassageLandsInOpenSpace` passed in `step2d_functional_23` for its seven-passage fixture.
+`RedefinedFieldTraversalSeeds` passed in `step2d_functional_24`: over three seeds, 21 authored
+passage landings bracketed open air and 18 arrival-to-exit walking routes were checked; six
+routes used a resolved coarse endpoint only after a full-density segment check.
+
+## Slices
+
+Each image places the old field on the left and the redefined field on the right. Light is air,
+dark is solid, and cyan marks the near-zero surface.
+
+- [Horizontal slice through the ring center](</E:/Projet Unreal/VoxelM/Plugins/VoxelForge/Saved/Research/step2d/step2d_rebuild_12/slice_xy_center.png>)
+- [Vertical slice at Y=0](</E:/Projet Unreal/VoxelM/Plugins/VoxelForge/Saved/Research/step2d/step2d_rebuild_12/slice_xz_y0.png>)
+- [Vertical slice at X=0](</E:/Projet Unreal/VoxelM/Plugins/VoxelForge/Saved/Research/step2d/step2d_rebuild_12/slice_yz_x0.png>)
+- [Vertical slice through an authored passage](</E:/Projet Unreal/VoxelM/Plugins/VoxelForge/Saved/Research/step2d/step2d_rebuild_12/slice_xz_passage.png>)
+
+Machine-readable measurements are in [step2d_report.json](</E:/Projet Unreal/VoxelM/Plugins/VoxelForge/Saved/Research/step2d/step2d_rebuild_12/step2d_report.json>).
+
+## Scaling to 12.5 cm and compile risk
+
+At 12.5 cm, double macro lengths and broad/medium noise periods in lattice units to preserve
+their world-space shape: the current noise periods 8/4/2/1 become 16/8/4/2 samples. Keep or add
+a 1-sample fine band so detail stays at least as rich as the 25 cm version. For a fixed world
+region, the lattice grows 8×, while surface-intersecting 8³ bricks should grow closer to 4×
+(surface area); remaining bricks still need only a cheap final-field sign proof and density
+writes. This scaling model is reasoned from the geometry and has not yet been timed at 12.5 cm.
+
+The UE 5.7 Win64 Development build passed with `-ForceHeaderGeneration` and
+`-MaxParallelActions=4`. No UCLASS/UPROPERTY layout changed, so no generated-class layout risk
+was introduced. The sensitive code is the locally bounded landing-floor derivative and the
+supremum used by `TryClassifyBrick`; future field terms must update those bounds and keep the
+proof audit. The live path and ARCHITECTURE §8.10 were left unchanged when research mode is off.
+No `.uasset` file was edited. Nothing was committed, pushed, or stashed.
+
+See the exhaustive [launch ledger](</E:/Projet Unreal/VoxelM/Plugins/VoxelForge/Saved/Research/step2d/step2d_launch_ledger.md>) for every benchmark, test run, failure, and prelaunch rejection.