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

COMPOSER-NOTES.md

@@ -840,6 +840,75 @@ capture allocation was **9,338,880 bytes** and the peak single RGBA raster was *
 full StructureRoll test completed in **243.649 s**, with 64 coarse filled pairs, 64 coarse contour
 pairs, 42 fine filled pairs, 42 fine contour pairs, and 0 refusals.
 
+### ✅ Morning archetype showcase + walkable floor-area audit (2026-09-05)
+
+The owner-facing review artifact is Saved/VoxelForge/Showcase/index.html. It is generated only by
+the editor automation test VoxelForge.Composer.Showcase; opening it is offline and has no runtime
+generation hook. It contains one alphabetized card for each native archetype, with the exact four PIE
+hand-off values, recipe, coarse step-4 filled/contour XZ+XY views, and a separate step-1 filled/contour
+ROI centred on that card's coarse LargestComponentPoint. The fine ROI is bounded at radius 64 and
+MaxCells=2,000,000; a refusal or blank ROI is written as text rather than represented by an empty
+image. All eight cards in the final run rendered both fine views; refusals **0**, blank cards **0**.
+
+The Part A table adds three measurements to the original volumetric walkability ratio:
+
+* WalkableFloorAreaFraction = distinct sampled XY columns containing at least one walkable cell /
+  the sampled XY footprint. It is an area/footprint proxy, not a continuous world-area integral.
+* MedianVerticalClearance is the lower median of the sampled air run above walkable cells, reported
+  in voxels. Both the walkability predicate and this median use the stated sample step.
+* LargestWalkableSurfaceShare is the largest 4-neighbour connected component of the projected
+  walkable columns / all walkable columns. It is a 2D surface-continuity indicator, not a proof that
+  floors at different Z values connect in 3D; the separate exact unsnapped arrival→departure route
+  check remains the traversal gate.
+
+The showcase selection exhausts seeds **0** and **7331**, candidate indices **0–63**, and excludes
+multi-region rolls while VF_LateralRegionsAreShippable() is false. The hard gates are non-vacuous,
+largest air component share ≥ **0.50**, and exact unsnapped arrival→departure connectivity. Among
+those survivors the deterministic score is floor_area + 0.25*clamp(clearance/64,0,1) +
+0.10*largest_surface; floor area is primary, clearance and projected continuity are tie-breakers.
+The fixture and PIE hand-off use the project generator seed **0**, bComposerRollStructure=false,
+and leave the existing slot layout, passage endpoints, placement, and WorldRadiusVoxels=0 intact.
+
+Selected-card measurements (each card prints its own exact derived-margin Z window; common XY window
+is [-256,256) × [-256,256) voxels, step **4**, HeadroomCells=2, MaxCells=8,000,000):
+
+| Archetype | walkable fraction | floor area fraction | median clearance | largest surface share |
+|-----------|------------------:|--------------------:|-----------------:|----------------------:|
+| CrystalChamber | 0.059105 | 0.999878 | 68 | 1.000000 |
+| FlatPlain | 0.056107 | 0.999878 | 72 | 1.000000 |
+| FloatingIslands | 0.018869 | 0.400146 | 36 | 0.678005 |
+| Maze | 0.286020 | 0.290466 | 8 | 0.990544 |
+| SurfaceWorld | 0.064233 | 0.937378 | 64 | 0.993619 |
+| TunnelNetwork | 0.155047 | 0.208374 | 20 | 0.463972 |
+| Underwater | 0.228995 | 0.206055 | 12 | 0.331161 |
+| VerticalShafts | 0.108252 | 0.113770 | 12 | 0.144850 |
+
+The result is mixed, and that is the finding. CrystalChamber, FlatPlain, and SurfaceWorld retain
+roughly **0.94–1.00** walkable XY footprint while their volumetric walkable ratios are only
+**0.056–0.064**: the original ratio was primarily measuring corridor occupancy and was the wrong
+quality target for broad slab/chamber floors. Maze is genuinely corridor-rich: its ratio and floor
+area are both about **0.29**, and **0.991** of its projected walkable surface is in the largest
+component. Conversely, TunnelNetwork, Underwater, and VerticalShafts have only **0.114–0.208** floor
+area and **0.145–0.464** largest-surface share; FloatingIslands is intermediate (**0.400** area,
+**0.678** largest share). Those are real fragmentation/ledge findings, not just a strict-volume
+artifact. All selected cards pass the separate mouth-to-mouth route check, so “walkable area” still
+does not mean every walkable patch is on the traversable route.
+
+The web sanity check did not produce a defensible universal “good” floor-to-volume threshold. The
+official NPS descriptions instead support a heterogeneous target: solution caves grow along connected
+fractures and streams, passages branch and join, and caves can contain multiple levels, narrow
+walking passages, rooms, and vertical pits. NPS's Lehman Cave dimensions give a useful scale example
+(the Talus Room is reported as 90 ft wide × 376 ft long, with 21,511 sq ft of floor area and 113 ft
+floor-to-ceiling), not a universal ratio. The official Deep Rock Galactic cave-design note likewise
+describes rooms connected by narrow winding tunnels, while the Team Cherry map-design interview
+emphasizes interconnected spaces and backtracking. References: NPS Solution Caves
+(https://www.nps.gov/subjects/caves/solution-caves.htm), NPS Mammoth Cave morphology
+(https://home.nps.gov/maca/learn/nature/how-mammoth-cave-formed.htm), NPS Lehman Cave dimensions
+(https://www.nps.gov/grba/learn/nature/lehman-caves-dimensions.htm), Deep Rock Galactic procedural
+cave design
+(https://store.steampowered.com/news/posts/?appgroupname=Deep+Rock+Galactic&appids=548430&enddate=1729500950&feed=steam_community_announcements),
+and Team Cherry map design (https://www.pcgamer.com/how-to-design-a-great-metroidvania-map/).
+
 ### 3.5 Validation in three layers, and how "good" is ever judged
 
 1. **Construct by design** so the law cannot break — build the connections FIRST and generate around