feat(composer): showcase of all 8 archetypes -- and "not navigable" answered

Jahni asked for one strate per archetype to look at, and said the previews
looked "not navigable nor anything except maybe the mazes". Both delivered.

⭐ PART A -- HIS EYE WAS RIGHT, AND SO WAS THE METRIC. BOTH EXPLANATIONS
WERE TRUE, and they split by archetype:

  archetype        walkable   FLOOR AREA   largest connected surface
  CrystalChamber     .059        .9999          1.00
  FlatPlain          .048        .9985           .9998
  SurfaceWorld       .064        .9374           .9936
  Maze               .285        .2629           .9905
  Underwater         .229        .2061           .3312
  TunnelNetwork      .155        .2084           .4640
  VerticalShafts     .108        .1138           .1449
  FloatingIslands    .004        .1016           .4992

1. THE METRIC WAS WRONG FOR CHAMBERS. Walkable fraction is volumetric: an
   air cell with solid one step below and headroom above. In a cathedral
   almost no air cell is near a floor, so a huge open room scores ~.06 while
   a tight maze scores ~.29. It was measuring CORRIDOR-NESS, not
   navigability -- and mazes win that by construction, which is exactly the
   pattern he spotted. Slab families actually have 94-100% walkable FLOOR
   AREA, fully connected. They were never the problem.

2. AND FOUR ARCHETYPES ARE GENUINELY FRAGMENTED, which is a real content
   finding, not an artifact. Largest connected walkable surface:
   VerticalShafts .14, Underwater .33, TunnelNetwork .46, FloatingIslands
   .50. In VerticalShafts, 86% of the floor is unreachable from the biggest
   patch. The primordial law still passes -- you CAN get from arrival to
   departure -- but most of the floor is not part of that. That is precisely
   "not navigable" and it needed a new metric to see.

New metrics: WalkableFloorAreaFraction, NumWalkableSurfaceComponents,
LargestWalkableSurfaceShare. A floor you cannot cross is not a floor.

Web references checked, per his suggestion -- we invented every metric in
this project without once testing them against reality: NPS solution-cave
and Mammoth Cave morphology, Lehman Cave dimensions (Talus Room 90x376 ft,
21,511 sq ft floor, 113 ft ceiling), Deep Rock Galactic cave-design notes,
Team Cherry on Metroidvania maps. They support heterogeneous room+passage
space with branches and shafts -- NOT a universal floor-to-volume target.
So there is no single number to optimise, which is itself worth knowing.

PART B -- Saved/VoxelForge/Showcase/index.html, 8 cards, 64 PNGs, fine
renders (step 1, radius 64, centred on LargestComponentPoint), 0 blanks.
Each card carries the exact PIE values to type in, because navigability is
judged by walking:

  CrystalChamber  seed 7331 idx 34 slot 1     Maze            seed 0 idx 58 slot 1
  FlatPlain       seed 0    idx  4 slot 1     SurfaceWorld    seed 0 idx 11 slot 2
  FloatingIslands seed 0    idx 34 slot 1     TunnelNetwork   seed 0 idx 61 slot 1
  Underwater      seed 0    idx 46 slot 1     VerticalShafts  seed 0 idx 15 slot 1

All bComposerRollStructure = false. Selection searched seeds 0/7331 x
indices 0-63, applied the three hard gates, then scored
floor_area + .25*clearance + .10*largest_surface.

Caveats kept honest: floor area is a sampled XY proxy, projected
connectivity is not a 3D traversal proof, "best" means best within the
searched range.

Build clean, 30 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

@@ -330,7 +330,9 @@ When both `CoverPointA` and `CoverPointB` are set, the XY window is their AABB e
 `CoverMarginVoxels`; otherwise it is the legacy square centered on `CenterXY` with
 `RadiusInVoxels`.
 It performs one deterministic 6-connected air flood fill, and derives fractions, components,
-walkability, feature scale, and clearance from that grid. Metrics report the resolved margin and
+walkability, feature scale, and clearance from that grid. It then performs one bounded 4-neighbour
+flood fill over the projected XY columns that contain a walkable surface; this is a floor-continuity
+indicator, not another source-density sample or a 3D route proof. Metrics report the resolved margin and
 the inclusive/exclusive voxel Z window used, the exact sampled dimensions/bounds, and the largest
 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,
@@ -354,7 +356,7 @@ has a full-resolution air-verified route, and the query reports the alternate ro
 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. |
+| `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, deterministic air-component facts, walkable floor-area columns/fraction, projected surface components/share, and median clearance. |
 | `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. |
@@ -535,9 +537,11 @@ and scalar density=0 contour views side by side without blurring the sampled ras
 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 caller centres it on each coarse survivor's `LargestComponentPoint` and the writer labels it
-“largest open space” with its exact ROI and step; cap refusals are recorded 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.
+“largest open space” with its exact ROI and step; cap refusals and blank fine ROIs are recorded explicitly
+instead of showing empty evidence. The ordinary 64-card sheet remains sorted by survivor status then
+descending corpus-centroid distance. The `VoxelForge.Composer.Showcase` test additionally writes an
+alphabetized one-card-per-archetype PIE hand-off page with the Part A survivor distributions, exact seed /
+candidate / target-slot values, and no runtime generation hook.
 
 **`Public/VoxelSeasonManifest.h` + `Private/VoxelSeasonManifest.cpp`** — Tier 4c's offline season
 artifact. `VF_ComposeSeason` generates a bounded configurable candidate batch, applies the Tier 2
@@ -621,6 +625,7 @@ The plugin's first tests (`OPSTACK-PLAN.md` Phase 0.5). Run them from the editor
 | ″ | `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. |
 | `VoxelForgeComposerPromotionTest.cpp` | `VoxelForge.Composer.Promotion` | Re-measures the 12 project/default members, simulates five deterministic 24-candidate seasons with normalized measured-metric novelty (`<0.20`), cap 6, cumulative JSON promotion, provenance counts, corpus-hash checks, fresh-load gate verification, spread/survival reporting, and deliberate stale-metric corruption. Final run: **9 promoted**, corpus **4/8/9**, survival **20.8/54.2/54.2/45.8/50.0%**, spread **0.869214→0.916302**, **63.556 s**, 0 failures. |
 | `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 centred on `LargestComponentPoint`, checks exact arrival→departure connectivity, rerolls every recipe/stack for determinism, and brute-forces every uniform box verdict. Final run: **42/64 survival (65.6%)**, **64 distinct recipes**, **0 invalid recipes**, **64 coarse filled + 64 coarse contour pairs**, **42 fine filled + 42 fine contour pairs**, blank plan/card **6/42→3/42**, **243.649 s**, **0 refusals**. |
+| `VoxelForgeComposerShowcaseTest.cpp` | `VoxelForge.Composer.Showcase` | Exhausts the bounded parameter-roll set (seeds **0, 7331**, indices **0–63**), excludes multi-region rolls while the lateral gate is off, and measures every missing-family candidate in all six interior target slots at step 4 / radius 256 / `MaxCells=8,000,000`. Hard gates are non-vacuous, largest air share ≥ **0.50**, and exact unsnapped arrival→departure connectivity; selection score is floor-area fraction + clearance tie-break + projected-surface tie-break. It asserts roll/manifest determinism, density sign, zero `WorldRadiusVoxels`, bit-identical metric reruns, and writes one alphabetized card per archetype to `Saved/VoxelForge/Showcase/index.html`, with step-1 radius-64 filled/contour plan + vertical ROI images centred on `LargestComponentPoint`. |
 | `VoxelForgeComposerSeasonTest.cpp` | `VoxelForge.Composer.Season` | Composes a six-slot offline season from 24 structure candidates; asserts **19 hard-gate survivors**, **6 selected**, deterministic byte-identical JSON, absolute descent order, all selected primordial-law facts, and a selected-only review page with metrics/reasons/boss markers. Loads the JSON, rebuilds every selected stack, and compares **393,216** bounded density samples bit-for-bit. Rejection audit: **4 vacuous**, **1 primordial-law budget**, **13 policy-not-selected**. |
 | `VoxelForgeComposerCorpusFreeTest.cpp` | `VoxelForge.Composer.CorpusFree` | Shares each structure recipe across today's corpus blend, naive independent uniform rolls, and constraint-sampled rolls. The completed equal-arm run uses **16 candidates per arm** (256×3 and 64×3 were stopped before aggregate output for runtime), step 4 / radius 256 / `MaxCells=8,000,000`, fixed passage-law mouths, and 40 box probes per candidate. Result: **13/16, 7/16, 7/16** survival; survivor walkable means **0.066071, 0.012566, 0.012851** and feature-scale means **92.307693, 64.571426, 31.428572**. Box checks: **196/260,876**, **310/412,610**, **262/348,722** proved/voxels, **0 violations** in every arm. |
 | `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. |