feat(preview): fine ROI renders + density contour -- the caves look like caves

Implements Codex's own recommendation from its critique, after it settled
the owner's two observations with evidence:

  "hard walls"  -> THE RENDERER. rg (room graph) re-rendered at step 1 is
                   rounded and organic. The hard edges were 4-voxel
                   sampling drawn nearest-neighbour. Fifth time in this
                   project that an alarming result was the instrument.
  "like a maze" -> REAL AND CORRECT. lt (lattice) stays rectilinear at
                   step 1 because it IS a maze; 19 of 64 candidates use
                   that source.
  "noisy"       -> ALIASING. nr#22 speckle 7 solid / 1 air at step 4 ->
                   0 / 0 at step 1, broad shapes intact.

Built: an optional fine ROI pass at step 1, radius 64 (~1.8M cells, cap
2M), run only for the 42 coarse survivors, plus a deterministic density=0
CONTOUR rendered beside the filled view.

Both views are kept side by side on purpose. The filled view is honest
about what was sampled and carries the walkability evidence; the contour
is legible. Codex's line, and it is the right principle: anti-aliasing the
coarse image "would only cosmetically blur the evidence and could invent
geometry". Do not replace evidence with beauty.

Fine labels carry their own sample step and resolved window so a fine
image can never be mistaken for a coarse one, and a MaxCells refusal shows
in the page rather than looking like a failed render.

  fine sampling 71.1 s · rendering 0.6 s · peak grid 9.34 MB
  42 filled pairs, 42 contour pairs, 0 refusals, 424 image links, 0 missing

Codex corrected my spec twice in the critique, both sharp: a binary step-1
raster ALWAYS has one-voxel axis-aligned edges, so only persistent
LARGE-SCALE right angles prove boxy generation (my criterion would have
produced a false "it is boxy" verdict); and persistent speckle is not
necessarily a roughness modifier.

Roughness deliberately untouched -- its call to judge that only after fine
rendering exists, and it stands.

⚠️ KNOWN, next: the fine ROI is centred on (0,0), so a candidate whose
geometry sits elsewhere renders blank (candidate 003 is a flat sheet with
no surface in the ROI). FVoxelStrateMetrics already carries
LargestComponentPoint -- centre the ROI there instead.

Build clean, 25 succeeded / 0 failed, verified by me.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VJiTcTSDNkKBtnNdiBtdaU

CODEMAP.md

@@ -309,7 +309,7 @@ This is **where terrain shape lives.**
 
 ### 3.6b Strate measurement — `Public/VoxelStrateMeasure.h` + `Private/VoxelStrateMeasure.cpp`
 Headless, read-only Tier 2 measurement pass. `VF_MeasureStrate` samples one strate between a
-boundary-seal-derived (or explicitly overridden, including explicit zero) interior margin into a bounded coarse grid.
+boundary-seal-derived (or explicitly overridden, including explicit zero) interior margin into a bounded grid.
 When both `CoverPointA` and `CoverPointB` are set, the XY window is their AABB expanded by
 `CoverMarginVoxels`; otherwise it is the legacy square centered on `CenterXY` with
 `RadiusInVoxels`.
@@ -319,8 +319,11 @@ the inclusive/exclusive voxel Z window used, the exact sampled dimensions/bounds
 component's deterministic lowest-cell representative point, cell count, and count of components
 holding at least 1% of the air. The flood fill also retains component cell counts in discovery order,
 so diagnostics can classify small non-largest components without a second source sample.
-Callers may optionally receive that exact polarity grid as `FVoxelStrateSampleGrid`; the editor-only
-composer preview consumes this hand-off rather than sampling the world again.
+Callers may optionally receive that exact polarity grid and scalar values as `FVoxelStrateSampleGrid`; the
+editor-only composer preview consumes this hand-off rather than sampling the world again. The filled view uses
+the captured `Air` polarity, while the paired density=0 contour uses the captured scalar field directly (no blur
+of the coarse raster). Capture refuses a grid over `MaxCells` before allocating it; ordinary metric callers do
+not retain either buffer.
 `VF_AreConnected` recovers a deterministic BFS parent path from the same kind of grid and checks
 candidate routes at full voxel resolution before returning `Connected`. When a candidate is
 refuted, its specific coarse cell edge is added to a deterministic array blocklist and BFS is
@@ -336,7 +339,7 @@ No UObject state, cache, actor, world, or PIE is required.
 | Symbol | Role |
 |--------|------|
 | `FVoxelStrateMeasureSettings` / `FVoxelStrateMetrics` | Plain settings/result structs for bounded strate sampling and derived measurements; callers may override the interior margin (including zero to sample the seal) or fit the XY window to two points with a margin, and results identify the resolved window/dimensions/Z range and deterministic component representatives/sizes. |
-| `FVoxelStrateSampleGrid` | Optional one-pass `density > 0` air/solid capture exported by the measurement functions; bounded by `MaxCells`, consumed by editor/automation previews, never retained on ordinary metrics calls. |
+| `FVoxelStrateSampleGrid` | Optional one-pass `density > 0` `Air` polarity plus exact scalar `Density` capture exported by measurement functions; bounded by `MaxCells`, consumed by editor/automation previews, never retained on ordinary metrics calls. |
 | `VF_MeasureStrate` | One-grid/one-flood-fill strate metrics; resolves either the legacy centered square or the two-point fitted AABB, and refuses invalid bounds or a grid over `MaxCells`. |
 | `VF_AreConnected` | Coarse 6-connected BFS plus deterministic blocked-edge retries, with a full-resolution air recheck for every candidate route and explicit endpoint-solid, out-of-window, `NotConnectedAtThisResolution`, `CoarseLiedBudgetExhausted` (unknown), and connected outcomes; diagnostics include endpoint component facts and the retry count from that same grid. |
 | `VF_DiagnoseConnectivity` | The same query plus each mouth's air-component size/share and exact geometric distance to the nearest cell of the other component; proximity is measured in coarse-cell coordinates and does not claim a route through solid. |
@@ -474,10 +477,13 @@ the offline sampler, and brute-forces every uniform box verdict from the novel s
 
 **`Public/VoxelStratePreview.h` + `Private/VoxelStratePreview.cpp`** — editor/automation-only PNG
 renderer and self-contained contact sheet. It consumes `FVoxelStrateSampleGrid` from the measurement
-pass, emits deterministic centre-Y XZ and data-selected XY slices via `IImageWrapper`, caps each
-image at 512 pixels (including its scale footer), and sorts the 64-card sheet by survivor status then
-descending corpus-centroid distance. The page repeats each candidate's exact measurement window,
-marks rejection reasons, explains the 2D-connectivity limitation, and has no runtime generation hook.
+pass, emits deterministic centre-Y XZ and data-selected XY slices via `IImageWrapper`, and shows filled-cell
+and scalar density=0 contour views side by side without blurring the sampled raster. Each image is capped at
+512 pixels (including its scale footer). `FVoxelStrateFinePreviewSettings` is a separate caller-selected
+fine pass: the composer selects only survivors at step 1, radius 64 (a 128×128 XY ROI), and `MaxCells=2,000,000`;
+the writer labels every image with its exact ROI and step and records cap refusals in the page. The 64-card
+sheet remains sorted by survivor status then descending corpus-centroid distance, repeats exact measurement
+windows, marks rejection reasons, explains the 2D-connectivity limitation, and has no runtime generation hook.
 
 ### 3.9 Player edits — `Public/VoxelDiffLayer.h` + `.cpp`
 `UVoxelDiffLayer : UObject` (h:77). Stores `FVoxelModification` (h:43: Center/Radius/Strength;
@@ -544,7 +550,7 @@ The plugin's first tests (`OPSTACK-PLAN.md` Phase 0.5). Run them from the editor
 | `VoxelForgeStrateParamCoverageTest.cpp` | `VoxelForge.Determinism.StrateParamBlendCoverage` | **The X-macro guard** (added 2026-08-17). `FStrateGenerationParams::Lerp` blends the hand-written `VF_STRATE_PARAM_FIELDS` list, **not** the struct — so a field added to one and not the other compiles, tests green, and silently takes its **default** inside every Gradient/Interleaved transition band. This expands the X-macro a **third** way (after LERP and SNAP), into a name list, and diffs it against the struct's UObject reflection. Pure shape test: no fixture, no world, instant. `GExemptFieldNames` is **empty** — every reflected field is covered today, and any exemption must be written down as a decision. Stakes rise with the world composer, which intends to invent parameter sets through this same `Lerp` (`COMPOSER-NOTES.md`). |
 | `VoxelForgeComposerParameterRollTest.cpp` | `VoxelForge.Composer.ParameterRoll` | Asset-Registry corpus audit + complete per-archetype spread/exclusion/clamp table; asserts bit-identical deterministic rerolls and same-archetype parents; measures 64 transient candidate strates with `VF_MeasureStrate` plus the exact unsnapped arrival→departure law; brute-forces every rolled production box verdict. Bootstrap follow-up: **1,422 applications / 168 distinct fields**; **38/64 survival**, feature scale **0→376**, walkable **0.000000→0.286766** (no broadening beyond the prior run); **1,356 Mixed + 608 AllSolid + 596 AllAir = 1,204 proved boxes**, **1,602,524 lattice voxels checked, 0 violations**, 146.250 s total in the final full-namespace run. |
 | ″ | `VoxelForge.Composer.TerrainDetailLiveness` | Fixed 4,096-point `GetDensityAt` lattice, legacy and operator-stack paths; changes one terrain-detail group at a time with an empty terrain-op pool. Proves 9 live groups / 23 fields and 3 dead groups / 11 fields; all 24 rows match. |
-| `VoxelForgeComposerStructureRollTest.cpp` | `VoxelForge.Composer.StructureRoll` | Rolls root polarity → legal shape source → polarity-derived conversion → 4–8 declaration-legal modifiers → mandatory structural posts; blends the six native parameter families independently, measures 64 novel stacks, captures the same grid for the deterministic XZ/XY preview, checks exact arrival→departure connectivity, rerolls every recipe/stack for determinism, and brute-forces every uniform box verdict. Focused run: **42/64 survival (65.6%)**, **64 distinct recipes**, **6,848 bootstrap applications / 107 distinct fields**, **659 proved boxes / 877,129 voxels / 0 violations**, **64 rendered pairs + index.html**. |
+| `VoxelForgeComposerStructureRollTest.cpp` | `VoxelForge.Composer.StructureRoll` | Rolls root polarity → legal shape source → polarity-derived conversion → 4–8 declaration-legal modifiers → mandatory structural posts; blends the six native parameter families independently, measures 64 novel stacks, captures the same grid for the deterministic filled/contour XZ/XY preview, runs a separate step-1 radius-64 ROI pass for the 42 survivors, checks exact arrival→departure connectivity, rerolls every recipe/stack for determinism, and brute-forces every uniform box verdict. Focused run: **42/64 survival (65.6%)**, **64 distinct recipes**, **6,848 bootstrap applications / 107 distinct fields**, **659 proved boxes / 877,129 voxels / 0 violations**, **64 coarse filled + 64 coarse contour pairs**, **42 fine ROI filled + 42 fine contour pairs**, **184.659 s**, **0 refusals**. |
 | `VoxelForgeLayoutOrderIndependenceTest.cpp` | `VoxelForge.Determinism.LayoutOrderIndependence` | Builds a known transient soft-pointer pool, then rebuilds it in original, reversed, and swapped orders. Requires a non-empty layout and passage set, and compares every slot's definition/Z/height plus passage endpoints, radius, type, control geometry, and bounds bit-for-bit. |
 | `VoxelForgePassageOpenSpaceTest.cpp` | `VoxelForge.Determinism.PassageLandsInOpenSpace` | Uses the real fixture density path to check every generated inter-strate passage whose destination query answers: a 16-point ring outside the mouth's carve/blend band has at least half its samples in destination air, and the endpoint matches the pure open-point result within the mouth's float envelope. This is a connectivity proxy, not a flood-fill proof. Reports checked passages and false/unanswerable archetypes; fails if it inspects zero passages. |
 | `VoxelForgeStrateConnectivityTest.cpp` | `VoxelForge.Generation.StrateConnectivity` / `VoxelForge.Generation.StrateConnectivityRefinement` / `VoxelForge.Generation.VerticalShaftSeamFreedom` | Bounded strate metrics with density-polarity and solid-gap controls, deterministic route rechecks, and refinement sweeps. The refinement test fits the measurement AABB to each arrival/departure mouth pair, reports exact before/after seed-6 cell counts, reruns every negative with doubled margin, measures tree orphan candidates/path reachability and roughness-bubble proxies, asserts axis landings, checks source/mirror physical paths, and requires 16/16 effective arrival→departure results across the 16 VerticalShafts seeds. Margin-binding negatives are reported as measurement limits, never as gap findings. The seam test re-evaluates cell-boundary positions after warming distinct neighbouring chunk contexts in both legacy and operator-stack paths. |