feat(showcase): add sea-stack-arch (nature)

The tenth piece in the nature category: a coastal diorama tile, 2.54 x
1.92 m and 1.62 m high. A fragment of a horizontally bedded sandstone
headland stands on a wave-cut platform: sixteen layers, hard beds 42-168
mm thick alternating with soft beds 20-56 mm, at one set of heights in
every column so the strata run on from the arch into the stack. The soft
beds are undercut behind the hard beds either side, the undercut coming
and going along a face (all but flush in places, deepest at each leg's
foot, the wave-cut notch). The fin is cut by twelve vertical joint faces
shared by both legs and the lintel; each bed sets each face back by its
own amount, drifting coherently with height, and vertical joints split the
beds into blocks that stand back by their own amounts, the joints opened
as narrow V's, with fallen-out blocks and chipped corners. The arch: two
legs whose beds step in toward the opening (0.50 m at the foot, 0.26 m
under the roof) under a lintel of four hard beds whose lowest is whole but
for the crown joint. A twelve-layer stack stands on its own island in the
bay. The platform carries a bay, a channel through the arch, two tide
pools and the same joint sets as cracks; the sea is a rippled sheet at
0.105 m with its rim run under the platform, foam lies at the waterline
round the bay and the island, and five fallen blocks and eight boulders
rest on the platform. Turf with sea grass and sea thrift (pink heads on
stalks) covers the tops. Eight materials; the littoral zones (weed,
barnacles, black lichen), orange lichen and bird lime are in the shaders.

Every column is sampled on one fixed set of rays from its centre; a bed's
outline on a ray is where the ray leaves the intersection of its
half-planes, each set in by the bed's set-back, so a soft bed is inside
both hard beds either side by construction.

Budgets (measured, 5.2.1): 78182 tris (77600-78800), LOD 0.5000/0.2200,
8 materials with face floors, UVs 0..1 no overlap, AABB 2.5440x1.9225x
1.6184 (+-0.01), collider 342 (<=380, five hulls so players pass under
the arch), hygiene 0 incl. 0 coplanar cross-shell pairs, zmin 0; beds
bite 0.0026-0.0142 m over 35 joints (0.002-0.040), the lintel's lowest
bed 0.0120 into each leg; stack mass centre 0.1220 m inside its foot
notch (>=0.050); legs and stack sealed 8/8 (worst 0.0373, >=0.020);
aperture 0.3680 wide x 0.7370 high (>=0.20 x 0.60); 18 bed tops level
(0.00 deg, <=0.8) and continuous (spread 0, <=0.004); 19 soft beds
undercut 0.0133-0.0290 (0.008-0.090); sea 0.10500 m, ripple 0.00238
(0.0015-0.0090), pools flat; rims 0.01226 under the platform (>=0.008);
foam 0.0043 off the waterline (<=0.006), land edge 0.0140 under the rock
(>=0.004); 13 loose rocks sealed (worst 0.0050, >=0.003), 0 overlaps;
289 roots 0.0078-0.0189 in turf or stone (0.004-0.060), none loose. Two
default runs identical; 4.5.11 and 5.1.2 print the same measurements
(export differs by 8 bytes).

Fixes found on the way: the first outlines cut the soft beds with planes
applied as insets along the base normal, which threw a lintel vertex to
y=-0.12 (bite -0.22); outlines are now true half-plane intersections
sampled on shared rays. Clipping by projection folded rings onto plane
corners (166 zero-area faces, 39 doubles). Early renders read as stacked
masonry: rounded slabs, every soft bed undercut all round; the joint faces
are now shared through all beds, the undercut patchy, the arrises 3.5 mm
and creased at 18 deg. The flat beds were a coplanar hazard (up to 448
pairs, then isolated soft-band/arris coincidences): each bed's walls
batter by their own degree or two and turn slightly, arrises carry a
sub-mm wear per ray, and a final pass draws any soft facet still in
another bed's plane 1.5 mm in and turns any blade or head 3 deg about its
root. The seal audit first read "flank" vertices in notched sectors and
failed a sealed foot; it now takes each sector's most-buried vertex. Pool
levels are read off the built platform (the analytic surface left a rim 4
mm over its water). Tile edge kept out of the tide-zone shaders.

Falsifiers (flag -> measured -> exit), 5.2.1 and 4.5.11 identical; tris
78182 and envelope unchanged in all:
  --skip-decimate  LOD1 1.0000                                    9
  --stray-vert     loose_v 1                                     15
  --lift-z         zmin 0.05000                                  16
  --gap-lintel     left leg under the lintel -0.0150 m           17
  --lean-stack     mass centre 0.0048 m inside footprint         19
  --cut-pillar     worst sector -0.0269 m, right leg 0/8         20
  --close-arch     width 0.0000 m, height 0.4490 m               21
  --tilt-beds      2.536 deg                                     22
  --flush-beds     undercut 0.0061-0.0068 m                      23
  --flat-sea       ripple 0.00000 m                              24
  --short-sea      rim -0.05304 m                                25
  --lift-foam      0.02430 m off, land edge -0.00597 m           26
  --perch-talus    worst sector -0.0393 m                        27
  --pile-talus     3 pairs overlap                               28
  --float-cover    roots -0.0173 m, 315 loose                    29
tests/check_falsifier_targets.py --run --only sea-stack-arch passes.

Blender: E:\Blender-Developer-Tools\.scratch\5.2\blender-5.2.1-windows-x64\blender.exe,
reports "Blender 5.2.1 LTS"; default and every falsifier also run on
.scratch\blender-4.5.11-windows-x64 ("Blender 4.5.11 LTS"); default on
.scratch\blender-5.1.2-windows-x64 ("Blender 5.1.2").

Render: render_hero.py wrote hero and preview; framing fill x 0.625, y
0.861, margins left 0.172 right 0.203 bottom 0.028 top 0.111;
asset-quality floors pass (8 materials, edge90 0.033). Default stage, no
yaw; camera 13 deg left of the arch's axis so the opening reads against
the wedge on the wall.

Asset sheet (docs/gallery/asset-sheets/sea-stack-arch.webp): not the
least-designed object; the hydrant is plainer. Isolated, it reads as a
bedded rock arch and stack with water, foam and flowering tops. Weak
point: the stepped legs under a thick roof bed still lean toward a
stylised, tidy geometry rather than a ragged natural cliff.

Contact sheet (docs/gallery/contact-sheets/sea-stack-arch-contact-sheet.webp):
luma 0.2384 against 0.2369-0.2428 (in band, after relighting from 0.2852,
0.2550, 0.2209, 0.2334, 0.2357); stage 0.2125 against 0.1919-0.2517;
wedge warmth +0.0451 (calibration -0.144..+0.105; +0.1525 before the wedge
was raised off the floor); saturation 0.2073 against 0.1525-0.2367;
subject fill 0.861 vertical. Thumbnail legible: an arch and a stack over
blue water read at a glance.

Also updated: README, CLAUDE.md, AGENTS.md counts (66 -> 67), ROADMAP
Nature list (sea stack & rock arch shipped), asset_sheet.py SELECT row,
catalog.json, plugin.json and the regenerated gallery. build_gallery,
check_site_links (122 pages, 0 failures), check_falsifier_targets (static
and --run), check_exit_code_readme, check_engine_id, check_gallery_images
and check_import_export_rules pass.

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

Author: TMHSDigital

.cursor-plugin/plugin.json

@@ -204,6 +204,7 @@
     "showcase/go-kart",
     "showcase/broadleaf-oak",
     "showcase/farm-tractor",
-    "showcase/fern-mossy-rock"
+    "showcase/fern-mossy-rock",
+    "showcase/sea-stack-arch"
   ]
 }

AGENTS.md

@@ -21,7 +21,7 @@ classifies it as a `cursor-plugin`. This is content the AI loads when the user
 asks Blender questions or works on Blender add-ons in Cursor or Claude Code.
 
 The content base is 16 skills, 9 rules, 3 templates, 27 snippets, 60
-examples, and 66 showcase pieces (counts are CI-enforced against README.md)
+examples, and 67 showcase pieces (counts are CI-enforced against README.md)
 and the manifest). The full inventory tables and per-item purposes live in
 `CLAUDE.md`. Example anatomy and authoring rules: copy `examples/bmesh-gear/`;
 showcase conventions: `showcase/README.md`. The render look is specified

CLAUDE.md

@@ -22,7 +22,7 @@ rules/<rule-name>.mdc          - Anti-pattern rules, 9 total
 templates/<template-name>/     - Starter projects, 3 total
 snippets/<snippet-name>.py     - Standalone code patterns, 27 total
 examples/<name>/               - Runnable smoke-gated examples, 60 total (+ gallery.json)
-showcase/<name>/               - Budget-conformance props, 66 pieces, each with a gallery `category` (sibling of examples/; see showcase/README.md § Categories)
+showcase/<name>/               - Budget-conformance props, 67 pieces, each with a gallery `category` (sibling of examples/; see showcase/README.md § Categories)
 scripts/build_gallery.py       - Regenerates docs/gallery/ from examples/gallery.json + showcase/gallery.json
 scripts/site/                  - Vendored landing-page build (Jinja2); tokens.css is the shared palette
 tests/check_site_links.py      - Internal link/anchor/alt gate over the built site (docs/)

README.md

@@ -18,7 +18,7 @@
 </p>
 
 <p align="center">
-  <strong>16 skills</strong> &nbsp;&bull;&nbsp; <strong>9 rules</strong> &nbsp;&bull;&nbsp; <strong>3 templates</strong> &nbsp;&bull;&nbsp; <strong>27 snippets</strong> &nbsp;&bull;&nbsp; <strong>60 examples</strong> &nbsp;&bull;&nbsp; <strong>66 showcase pieces</strong>
+  <strong>16 skills</strong> &nbsp;&bull;&nbsp; <strong>9 rules</strong> &nbsp;&bull;&nbsp; <strong>3 templates</strong> &nbsp;&bull;&nbsp; <strong>27 snippets</strong> &nbsp;&bull;&nbsp; <strong>60 examples</strong> &nbsp;&bull;&nbsp; <strong>67 showcase pieces</strong>
 </p>
 
 <p align="center">
@@ -37,7 +37,7 @@
 
 ## Overview
 
-This repository ships **16 skills, 9 rules, 3 templates, 27 snippets, 60 examples, and 66 showcase pieces** for Blender Python development targeting Blender 5.2 LTS (current stable) with Blender 4.5 LTS fallback support. Blender 5.1 is prior stable.
+This repository ships **16 skills, 9 rules, 3 templates, 27 snippets, 60 examples, and 67 showcase pieces** for Blender Python development targeting Blender 5.2 LTS (current stable) with Blender 4.5 LTS fallback support. Blender 5.1 is prior stable.
 
 The content is consumed by AI coding agents reading these files directly from a checkout — **there is no MCP server in this repository, and none is required**. Cursor applies `rules/*.mdc` automatically wherever their scope globs match and takes skills by name in chat; Claude Code reads `skills/` and `rules/` from the project workspace, or from this repo kept as a referenced checkout. Any agent that can read files in a workspace can use it the same way. There is no build step for the content — edit the Markdown and Python files directly.
 
@@ -106,7 +106,7 @@ per-script exit-code model, are in
 Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](showcase/README.md).
 
 <details>
-<summary><strong>66 showcase pieces</strong> — click to expand the preview grid</summary>
+<summary><strong>67 showcase pieces</strong> — click to expand the preview grid</summary>
 
 <p align="center">
   <a href="showcase/shipping-crate/"><img src="showcase/shipping-crate/preview.webp" width="24%" alt="Shipping crate: a wooden slat crate stencilled PORT ROYAL and NO 17, with iron corner brackets and an end handle, on a dark studio floor, warm wedge on the back wall" /></a>
@@ -175,6 +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 broad oak with a short fluted trunk and crooked spreading limbs under a domed crown of lobed green leaves, on a grassy soil disc with surface roots, tufts, fallen leaves and acorns" /></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 finely cut fronds, with curled fiddleheads, in front of a cleft boulder cushioned in moss on a soil bank with fallen beech leaves, pebbles and wood sorrel" /></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>
 </p>
 
 [`shipping-crate`](showcase/shipping-crate/) — procedural crate through UVs, bake, LOD, collider, and Unity glTF, asserting recomputed budgets. Falsifier `--skip-decimate` exits 9.
@@ -305,6 +306,8 @@ 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 cleaved into broken, weathered faces and split by a deep crack, sunk into the bank all round, a thick moss cushion over its crown draped down its shaded front as a conforming shell with a lumpy rim tucked into the stone, 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 rooted fronds and rootstock, moss thickness over the stone, 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. `--float-fronds` and `--lift-crown` exit 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, `--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.
+
 </details>
 
 ## Examples