fix(showcase): remodel sea-stack-arch as one eroded headland mass in a sea that fills the tile

The arch read as tidy stacked masonry: rectangular blocks on a grid of
joints, square-cut stepped legs under a flat slab lintel. The sea was a
turquoise pond in a stone ring on a round pedestal, and the tops carried a
sparse, potted-plant turf. Rebuilt:

- rock: one mass of eighteen level beds (hard 65-132 mm, soft 18-35 mm) at
  one set of heights, legs 0-10, roof 11-17, stack 0-13. Each bed's outline
  is fourteen jointed faces turning round the fin, each leaning back with
  height, battered out below 0.64 m and flared at the foot, set back by the
  bed's own retreat. One weathering field in space (0.95/0.48/0.24 m swells)
  is read on the weathered face itself by a short fixed-point iteration, so
  a face runs on through the beds and from the legs into the roof whichever
  column's ray finds it. Per bed: proud/sunk offset, fallen-out blocks
  (pockets), a few open joints, a worn top arris and chamfered foot; soft
  beds undercut 13-79 mm, deepest at the wave-cut notch. The joint grid,
  the rectangular plan and the stepped cut planes are gone
- arch: the legs' inner faces curve in on a flattened ellipse from 0.55 m
  apart at the springing and meet (18 mm overlap) at the crown under the
  roof, their arrises cut back 38 deg; the roof's lowest bed takes the legs'
  top-bed face retreats so it does not overhang them; the upper roof beds
  step down seaward. The legs' top soft bed is also cast against the roof's
  lowest bed from the leg's own centre
- stack: a rugged tapering pillar standing in the sea on a flared,
  notched foot, its upper beds broken back on three sides
- tile: a rounded square, 2.84 x 1.96 m; the sea fills the right side to
  the edge, held by a 16 mm rim, and runs through the arch to the back
  edge; a wave-cut platform with two tide pools and wandering joint cracks,
  a shingle cove (Voronoi pebbles in sand, wet at the water) with 46
  pebbles, a straight skirt showing the beds in section; seabed deepens
  away from the shore so the water darkens; ripple calmed at the shore, the
  rim and round the feet
- foam: a line along the shore rim to rim, and loops round the leg and the
  stack standing in the water, land edges under the shore or in the rock
- tops: a turf mat draped over the edge as an overhanging lip, tucked into
  the stone, noise tussocks (not paving cells); thrift as leafy cushions
  with pink heads on short stalks; bird lime splashed on the high ledges and
  streaked down the seaward faces; strata tinted bed by bed
- dead helpers flank_bury, FLANK_R, FACETS, poly_normals removed

Budget changes (5.2.1):
- Base tris 77600-78800 -> 89900-90900 (90422); face floors re-fit
  (46448 sandstone, 13438 shelf, 5546 water, 1966 foam, 2856 cobble, 5540
  turf, 5331 grass, 3552 thrift measured); collider <= 380 -> <= 270 (244)
- Outer AABB -> (2.8400, 1.9600, 1.4649) m
- Beds bite 0.002-0.040 -> 0.001-0.030 over 41 joints (0.0019-0.0163; roof
  on the legs 0.0149, 0.0156). The minimum is the distance to the host's
  nearest face, which turns in beside a fallen-out block; parity is now a
  majority of seven rays (three miscounted at a pocket edge)
- Stack plumb >= 0.050 -> >= 0.100 (0.1276); aperture >= 0.20 x 0.60 ->
  >= 0.22 x 0.68 (0.264 x 0.757)
- Undercut 0.008-0.090 -> 0.008-0.085 (0.0125-0.0786), now read at each
  host's face ring nearest the soft bed rather than its mid-height, where
  the face's own lean and weathering swamped the recess
- Cover roots 0.004-0.060 -> 0.004-0.050 (0.0075-0.0395, 324 roots)
- Water contained 0.01007 (>= 0.008), foam 0.00430 off / 0.00535 tucked,
  feet sealed worst 0.0307 (>= 0.020), loose rock 57 sealed (worst 0.0050),
  0 overlaps, bedding tilt 0 / spread 0, 21 bed tops
- New exit 31, ragged: share of the rock's walls (faces within 20 deg of
  vertical, by area) whose plan normal lies within 5 deg of the column's
  axes <= 0.25 (0.1764)

Falsifiers (flag -> measured -> exit), 5.2.1 and 4.5.11 identical; tris
90422 and envelope unchanged in all (tidy-rock z 1.4585, within 0.01):
  --skip-decimate  LOD1 1.0000                                       9
  --stray-vert     loose_v 1                                        15
  --lift-z         zmin 0.05000                                     16
  --gap-lintel     left leg under the roof -0.0150 m                17
  --lean-stack     mass centre 0.0571 m inside footprint            19
  --cut-pillar     worst sector -0.0406 m, right leg 0/8            20
  --close-arch     width 0.2080 m (right side narrowed to a tenth)  21
  --tilt-beds      2.530 deg                                        22
  --flush-beds     undercut 0.0020-0.0065 m on failing beds         23
  --flat-sea       ripple 0.00000 m                                 24
  --short-sea      rim -0.01925 m                                   25
  --lift-foam      0.02430 m off, land edge -0.01465 m              26
  --perch-talus    worst sector -0.0420 m                           27
  --pile-talus     1 pair overlaps                                  28
  --float-cover    roots -0.0335..-0.0082 m, 398 loose              29
  --tidy-rock      ragged 0.8314 (new; boxes on the axes, lean and
                   batter kept, no weathering/blocks/joints/curve)  31
Fixes on the way: --close-arch first shifted the leg 0.30 m, which laid
its shared fin faces on the other leg's (exit 15); it now narrows the
leg's side of the hole. --lean-stack's shear flipped quad diagonals, so
beds are split by their unsheared corners. --tidy-rock first changed the
cover count (more clumps fit a box top, exit 4); cover is now spread over
the weathered top's outline in both runs. It then tripped the undercut
(exit 23) on the shallowest beds; the design undercut was deepened (26 mm
base, per-bed depth 0.75-1.45) rather than widening the band.
tests/check_falsifier_targets.py --run --only sea-stack-arch passes.

Evidence (live runs, local):
- .scratch/5.2/blender-5.2.1-windows-x64/blender.exe (Blender 5.2.1 LTS):
  default and all 16 falsifiers; two default runs identical; hygiene 0 incl.
  0 coplanar cross-shell pairs; export 9158084 B; default run ~44 s.
- .scratch/blender-4.5.11-windows-x64 (Blender 4.5.11 LTS): all 17 runs
  same codes and error values; measurements identical; glTF +8 B.
- .scratch/blender-5.1.2-windows-x64 (Blender 5.1.2): default exit 0,
  measurements identical.
- Framing: fill x 0.816 (was 0.625), y 0.861; margins l .119 r .066
  b .056 t .083; asset-quality floors pass.
- Contact sheet: luma 0.2401 (band 0.2369-0.2428), stage 0.2166 (<= 0.2517),
  warmth +0.0443 (spread -0.144..+0.105), sat 0.2133 (0.1525-0.2367).
  Thumbnail: rounded arch and stack over dark teal water at a glance.
- Asset sheet: before, a stacked-block ruin over a pond in a ring; now a
  ragged bedded arch and tapering stack in a sea that fills the tile, cove
  and platform beside. Not the least-designed object (the hydrant is
  plainer). Weak points: the beds still read as stacked plates up close;
  the skyline is flat under the turf; fallen blocks are boxy.
- build_gallery, check_site_links (122 pages, 0 failures), check_engine_id,
  check_exit_code_readme, check_falsifier_targets, check_gallery_images,
  check_import_export_rules pass.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: TMHSDigital

README.md

@@ -175,7 +175,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
   <a href="showcase/broadleaf-oak/"><img src="showcase/broadleaf-oak/preview.webp" width="24%" alt="Broadleaf oak: a mature oak with a massive buttressed trunk and crooked limbs reaching out under a broad dome of lumpy dark-green leaf cushions, on a grassy mound with roots, tufts and fallen leaves" /></a>
   <a href="showcase/farm-tractor/"><img src="showcase/farm-tractor/preview.webp" width="24%" alt="Farm tractor: green farm tractor in muddy ruts beside a puddle: long bonnet, barred grille, rusty exhaust stack, big lugged rear tyres on cream rims with weights, small ribbed fronts." /></a>
   <a href="showcase/fern-mossy-rock/"><img src="showcase/fern-mossy-rock/preview.webp" width="24%" alt="Fern and mossy rock: a male fern's arching crown of cut fronds and fiddleheads beside a split gritstone boulder with flat broken faces, lobed moss cushions and a small fern in its crack, on a soil bank with beech litter" /></a>
-  <a href="showcase/sea-stack-arch/"><img src="showcase/sea-stack-arch/preview.webp" width="24%" alt="Sea stack and arch: a layered sandstone sea arch on a wave-cut rock platform, a sea stack beside it on its own island, foam at the waterline, fallen blocks, and grass and pink sea thrift on the tops" /></a>
+  <a href="showcase/sea-stack-arch/"><img src="showcase/sea-stack-arch/preview.webp" width="24%" alt="Sea stack and arch: a ragged bedded sandstone sea arch and a tapering sea stack standing in a sea that fills the tile to its rim, beside a wave-cut platform and a shingle cove, with thrift on the turfed tops" /></a>
 </p>
 
 [`shipping-crate`](showcase/shipping-crate/) — procedural crate through UVs, bake, LOD, collider, and Unity glTF, asserting recomputed budgets. Falsifier `--skip-decimate` exits 9.
@@ -306,7 +306,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
 
 [`fern-mossy-rock`](showcase/fern-mossy-rock/) — the ninth `nature` piece: procedural shaded woodland corner on a sloping soil bank (a gritstone boulder cut by deep cleavage planes into broad flat fracture faces meeting at crisp, weathered arrises, with stepped ledges, pitting, lichen, a moss stain and a damp band at its foot, split down its front by a deep crack and sunk into the bank all round; lobed moss cushions and satellite tufts laid on it as conforming shells, deepest in its hollows, feathering to nothing at the rim, tucked over the crack's lip, with bare patches where the stone shows through; a male fern's shuttlecock crown of eleven bipinnate fronds whose 434 alternate pinnae are cut into pinnules and taper to the tip, three fiddleheads unrolling as logarithmic spirals from a scaly rootstock bedded in the soil, a smaller fern growing out of the crack, and beech litter, pebbles, a twig and wood sorrel at the foot), whose cleaved stone, rooted fronds and rootstock, moss thickness and feathered rim, boulder sealed in the bank, moss facing up or into the shade, crack fern wedged in the cleft, pinna alternation and taper, true spirals and ground cover are recomputed from the mesh, through the same pipeline. `--smooth-rock` exits 27 on the stone's turning concentrated in crisp arrises, `--float-fronds` and `--lift-crown` 17 on every frond rooted in its crown and the crown sealed in the soil, `--float-moss` 18 on the moss cushion's thickness band on the stone, `--slab-moss` 28 on the cushion feathering out at its rim, `--perch-rock` 20 on the boulder sealed all round, `--sunny-moss` 21 on moss facing up or into the shade, `--perch-crack-fern` 22 on the crack fern rooted in the cleft, `--flat-taper` and `--opposite-pinnae` 23 on pinnae alternating and tapering along the rachis, `--open-crozier` 24 on every fiddlehead a real spiral, `--float-cover` 25 on the ground cover joined to the soil, while the envelope holds.
 
-[`sea-stack-arch`](showcase/sea-stack-arch/) — the tenth `nature` piece: procedural coastal diorama tile (a fragment of a horizontally bedded sandstone headland on a wave-cut rock platform: sixteen hard and soft beds at one set of heights in every column, the soft beds undercut so the hard beds project as ledges, vertical joint faces shared by both legs and the lintel, blocks set back and fallen out between the joints; a natural arch of two stepped legs carrying a lintel over a channel; a sea stack of the same beds on its own island; a rippled sea at a declared level with its rim under the platform, foam at the waterline, two tide pools, fallen blocks and boulders, and turf with sea grass and sea thrift on the tops), whose beds biting each other and the lintel biting both legs, stack over its footprint, feet sealed in the platform, clear aperture, level continuous bedding, undercut soft beds, sea level and ripple, contained water, seated foam, loose rock sealed and apart, and rooted cover are recomputed from the mesh, through the same pipeline. `--gap-lintel` exits 17 on every bed biting the one it lies on, `--lean-stack` 19 on the stack's mass over its footprint, `--cut-pillar` 20 on the feet sealed all round, `--close-arch` 21 on the clear aperture, `--tilt-beds` 22 on level, continuous bedding, `--flush-beds` 23 on the undercut, `--flat-sea` 24 on the water level and ripple, `--short-sea` 25 on the water contained, `--lift-foam` 26 on the foam at the waterline, `--perch-talus` 27 and `--pile-talus` 28 on the loose rock sealed and apart, `--float-cover` 29 on the cover rooted in the tops, while the envelope holds.
+[`sea-stack-arch`](showcase/sea-stack-arch/) — the tenth `nature` piece: procedural coastal diorama tile (a fragment of a bedded sandstone headland at the sea's edge as one eroded mass: eighteen level hard and soft beds running on from the legs into the roof and the stack, jointed faces leaning back, battered and flared at the foot, one weathering field read on the face through every bed, fallen-out blocks and open joints, soft beds undercut to a wave-cut notch; a natural arch of two legs curving in to meet at the crown under a roof that steps down seaward; a tapering stack standing in the sea; the sea filling the tile to its rim, foam at the shore and round the feet, a wave-cut platform with tide pools, a shingle cove, fallen blocks, boulders and pebbles, and a draped turf mat with sea grass and thrift cushions), whose beds biting each other and the roof biting both legs, stack over its footprint, feet sealed in the ground, clear aperture, level continuous bedding, undercut soft beds, sea level and ripple, contained water, seated foam, loose rock sealed and apart, rooted cover and ragged (not squared) rock are recomputed from the mesh, through the same pipeline. `--gap-lintel` exits 17 on every bed biting the one it lies on, `--lean-stack` 19 on the stack's mass over its footprint, `--cut-pillar` 20 on the feet sealed all round, `--close-arch` 21 on the clear aperture, `--tilt-beds` 22 on level, continuous bedding, `--flush-beds` 23 on the undercut, `--flat-sea` 24 on the water level and ripple, `--short-sea` 25 on the water contained, `--lift-foam` 26 on the foam at the waterline, `--perch-talus` 27 and `--pile-talus` 28 on the loose rock sealed and apart, `--float-cover` 29 on the cover rooted in the tops, `--tidy-rock` 31 on the rock ragged rather than squared, while the envelope holds.
 
 </details>