fix(showcase): give wooden-ladder through-tenons, iron tie-rods and two-tone timber

Earlier passes only touched lighting, materials and the turned rungs, and the
bare ladder stayed the plainest object in its asset sheet: the exact 16-shell
parts budget kept every detail inside the existing shells. This pass changes
the model and raises the budgets, with the justification in the README.

- Two iron tie-rods run through both rails under the bottom and top rungs.
  Each end is clamped by a square nut over a round washer bedded on the
  rail's outer face (parts 16 -> 26).
- The rungs are through-tenoned: each tenon runs right through its rail and
  stands 2 mm proud. The old blind-tenon "breakout margin" becomes a
  through-tenon proud band (0.001-0.004 m). The tenon's inboard ring moves
  to 4 mm inside the rail, so the joint audit still samples tenon section
  inside the wood.
- A per-shell Species attribute: pale hickory rungs against darker oak
  rails. End grain (tenon ends, rail tops) reads darker. The iron's rust is
  darker, so the hardware reads as iron.
- The rung finders (rung_pitch, the render's rope coil) now require the wood
  material, because the tie-rods are as wide as a rung.
- New budget: tie-rod seat. It checks 2 rods, 4 washers and 4 nuts: washer
  into the stile and nut into the washer 0.0002-0.002 m, on the rod axis
  within 1.5 mm, and the thread >= 3 mm proud of each nut. New falsifier
  --float-nuts exits 20.

Budgets, before -> after:
- base tris 2680 -> 3000; ceiling 2800 -> 3100 (hardware +320)
- parts 16 -> 26
- metal faces 736 -> 832
- collider 24 -> 92 tris (ceiling 180)
- bbox (0.4897, 0.3760, 1.4870) -> (0.4903, 0.3760, 1.4870)
- tenon proud 0.00199, tie seat bite 0.00049..0.00051, thread 0.00450

Evidence (live, local):
- .scratch/blender-5.2.1-windows-x64/blender.exe (Blender 5.2.1 LTS) and
  .scratch/blender-4.5.11-windows-x64/blender.exe (Blender 4.5.11 LTS):
  default exit 0 with identical measurements.
- tests/check_falsifier_targets.py --run on both binaries: all 8 wooden-ladder
  falsifiers land (--skip-decimate 9, --stray-vert 15, --twin-sole 15,
  --lift-z 16, --fat-rungs 17, --short-stile 18, --drift-rungs 19,
  --float-nuts 20). The static check passes (338 falsifiers across 36 pieces).
  --float-nuts measured failure: seat bite -0.00350, thread 0.00050 proud.
- Framing on both versions: fill y 0.883, margins l/r 0.416, b 0.044, t 0.072.
- Contact sheet: luma 0.210, stage 0.201, warmth +0.086, saturation 0.173.
- Asset sheet (docs/gallery/asset-sheets/wooden-ladder.webp): the tie-rods,
  the two-tone rungs and the tenon ends now read. It is on a par with the
  hydrant, not clearly the plainest, though a ladder is still the thinnest
  silhouette in the lineup.
- 5.1.2 was not re-run.

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

Author: TMHSDigital

README.md

@@ -133,7 +133,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
   <a href="showcase/wall-torch/"><img src="showcase/wall-torch/preview.webp" width="24%" alt="Wall torch: a dressed stone plaque with an iron plate and arm holding a cup, a leaning wooden torch with a pitch-wrapped head and a bright flame, on a dark studio floor" /></a>
   <a href="showcase/tavern-stool/"><img src="showcase/tavern-stool/preview.webp" width="24%" alt="Tavern stool: a dished round wooden seat holding a pewter tankard, on turned splayed legs with stretchers at two heights and iron ferrules on flat treads, on a dark studio floor" /></a>
   <a href="showcase/iron-cauldron/"><img src="showcase/iron-cauldron/preview.webp" width="24%" alt="Iron cauldron: a round-bellied cast-iron pot hanging by its bail from a timber tripod on a dark studio floor" /></a>
-  <a href="showcase/wooden-ladder/"><img src="showcase/wooden-ladder/preview.webp" width="24%" alt="Wooden ladder: a raked timber ladder with six evenly spaced turned rungs and iron shoes, leaning on a weathered barn-board wall with a coil of hemp rope hung over a rung" /></a>
+  <a href="showcase/wooden-ladder/"><img src="showcase/wooden-ladder/preview.webp" width="24%" alt="Wooden ladder: an oak ladder with six pale turned hickory rungs through-tenoned into its rails, iron tie-rods under the end rungs and iron shoes, leaning on a barn-board wall with a rope coil on a rung" /></a>
   <a href="showcase/hay-bale/"><img src="showcase/hay-bale/preview.webp" width="24%" alt="Hay bale: a bound straw bale with two sisal twine belts on a dark studio floor" /></a>
   <a href="showcase/crate-stack/"><img src="showcase/crate-stack/preview.webp" width="24%" alt="Crate stack: three slatted shipping crates stacked and yawed, each with iron corner straps and runners, on a dark studio floor" /></a>
   <a href="showcase/stone-archway/"><img src="showcase/stone-archway/preview.webp" width="24%" alt="Stone archway: a semicircular masonry arch with coursed piers, projecting imposts, nine voussoirs and a proud keystone, on a dark studio floor" /></a>

docs/gallery/asset-sheets/wooden-ladder.webp

@@ -326,7 +326,7 @@
       "name": "wooden-ladder",
       "dir": "showcase/wooden-ladder",
       "teaches": "A procedural timber ladder with raked tapered stiles, six turned rungs, ferrule caps, and level shoe plates through UVs, bake, LOD, collider, and Unity glTF, asserting recomputed budgets rather than an API contract.",
-      "alt": "A timber ladder with tapered stiles and six evenly spaced lathe-turned rungs, leaning on a weathered board wall with a coil of hemp rope hung over its second-highest rung.",
+      "alt": "An oak ladder with six pale turned hickory rungs through-tenoned into its rails, iron tie-rods under the end rungs, leaning on a board wall with a rope coil on a rung.",
       "witnessesFix": "Recomputed: 2536 tris, two materials with 612 wood and 736 metal faces, UVs in 0..1 with zero AABB overlap, outer AABB 0.490×0.376×1.487 m, zmin 0, hygiene and coplanar pairs 0, tenon clearance 8 mm, shoe bite 20 mm, rung pitch even to 0.002 m, LOD ratios in band, convex collider 24 tris, non-empty glTF. --skip-decimate exits 9; --stray-vert and --twin-sole exit 15; --lift-z exits 16; --fat-rungs exits 17; --short-stile exits 18; --drift-rungs exits 19.",
       "hero": "docs/gallery/assets/wooden-ladder-hero.webp",
       "preview": "showcase/wooden-ladder/preview.webp",

docs/gallery/assets/wooden-ladder-hero.webp

@@ -1,10 +1,13 @@
 # Wooden ladder
 
 A showcase piece, not an example. Procedural timber ladder: two stiles
-tapered from 64 mm to 50 mm, raked 12°, six turned rungs (22 mm tenon, a
+tapered from 64 mm to 50 mm, raked 12°, six turned hickory rungs (a
 barrel that shoulders in just under its radius and swells 14 % at
-mid-span, the profile a lathe leaves on a ladder round), iron ferrules at the top, and
-level shoe plates under sleeves that follow the rake. Then the shipped
+mid-span, the profile a lathe leaves on a ladder round) through-tenoned
+into oak rails with their end grain showing 2 mm proud of the outer faces,
+two iron tie-rods under the bottom and top rungs clamped by square nuts
+over round washers, iron ferrules at the top, and level shoe plates under
+sleeves that follow the rake. Then the shipped
 pipeline: unique-cell UVs, Cycles high-to-low normal bake, LOD chain,
 convex collider, Unity glTF export.
 
@@ -37,24 +40,25 @@ imported). Coplanar-pair counting matches `showcase/signpost`.
 Declared as named constants; every gate **recomputes** from the mesh,
 materials, UVs, evaluated LOD, collider, or export file.
 
-| Axis | Declared | Measured (4.5.11 / 5.2.1; 5.1.2 not re-run after the turned-rung pass) |
+| Axis | Declared | Measured (4.5.11 / 5.2.1; 5.1.2 not re-run after the tie-rod pass) |
 | --- | --- | --- |
-| Base triangles | 1800–2800 | 2680 / 2680 |
+| Base triangles | 1800–3100 | 3000 / 3000 |
 | LOD1 ratio | 0.32–0.62 of base | 0.5000 / 0.5000 |
-| LOD2 ratio | 0.10–0.35 of base | 0.2194 / 0.2194 |
-| Materials | exactly 2 distinct, ≥280 wood, ≥180 metal | 2 slots, 684 wood, 736 metal |
+| LOD2 ratio | 0.10–0.35 of base | 0.2200 / 0.2200 |
+| Materials | exactly 2 distinct, ≥280 wood, ≥180 metal | 2 slots, 684 wood, 832 metal |
 | UVs | in `0..1`, AABB overlap ≤ 1e-5 | in range, overlap 0 |
-| Outer AABB | (0.490, 0.376, 1.487) m ± 0.01 | (0.4897, 0.3760, 1.4870) |
+| Outer AABB | (0.490, 0.376, 1.487) m ± 0.01 | (0.4903, 0.3760, 1.4870) |
 | Grounded | `abs(zmin)` ≤ 1e-4, each sole at 0, rail end ≥ 0.012 m | zmin 0, sole 0, rail 0.02037 |
 | Mesh hygiene | non-manifold, loose v/e, doubles at 1e-5, zero-area, n-gons, coplanar pairs all 0 | all 0 |
-| Parts | 16 shells (2 stiles, 6 rungs, 2 sleeves, 2 cap sleeves, 2 plates, 2 soles) | 16 (2 / 6 / 2 / 2) |
+| Parts | 26 shells (2 stiles, 6 rungs, 2 sleeves, 2 cap sleeves, 2 plates, 2 soles, 2 tie-rods, 4 washers, 4 nuts) | 26 (2 / 6 / 2 / 2) |
 | Rung depth clearance | ≥ 0.003 m inside the stile faces | 0.00800 |
-| Tenon engagement | ≥ 0.008 m past the stile inner face | 0.01399 |
-| Tenon breakout margin | ≥ 0.008 m short of the stile outer face | 0.03787 |
+| Tenon engagement | ≥ 0.008 m past the stile inner face | 0.05387 (through) |
+| Through-tenon proud | tenon end 0.001–0.004 m past the stile outer face | 0.00199 |
 | Shoe bite / cover | bite ≥ 0.010 m, cover ≥ 0.016 m | 0.02000 / 0.03800 |
 | **Rung pitch** | 6 rungs; every rung-to-rung gap within 0.002 m of the mean | 6; 0.00000 |
-| Collider tris | ≤ 180 | 24 |
-| Export | written, size > 0 | 197604 / 197584 bytes |
+| **Tie-rod seat** | 2 rods, 4 washers, 4 nuts; washer into stile and nut into washer 0.0002–0.002 m; on the rod axis within 0.0015 m; thread ≥ 0.003 m proud of each nut | 2 / 4 / 4; bite 0.00049–0.00051; axis 0; thread 0.00450 |
+| Collider tris | ≤ 180 | 92 |
+| Export | written, size > 0 | 230268 bytes (5.2.1) |
 
 Base triangles rose from **936 to 2536**. The previous mesh was two
 boxes, six cloned dowels, and four chrome cubes. Turned rungs (tenon
@@ -75,6 +79,25 @@ across the stile's 50 mm width at a readable scale (`WOOD_GRAIN_SCALE`
 spanned a stile, so it rendered as flat tan. A short-range AO term darkens
 the rung housings and the wood under the iron sleeves.
 
+A third pass changed the model itself. The bare ladder was still the
+plainest object in its asset sheet, because the exact parts budget left
+every detail inside the existing shells. The parts budget is now **26**:
+two iron tie-rods run through both rails under the bottom and top rungs,
+each end clamped by a square nut over a round washer bedded on the rail's
+outer face. That is the rod a wooden ladder carries to stop the rails
+spreading off the rungs. The triangle ceiling rose **2800 → 3100** for the
+hardware (+320 triangles; the old ceiling had 120 to spare). The rungs are
+now through-tenoned: each tenon runs right through its rail and stands
+2 mm proud, so the old *breakout margin* (a blind tenon stopping short of
+the outer face) became a **through-tenon proud** band, and the tenon's
+inboard ring moved to 4 mm inside the rail so the joint audit always
+samples tenon section in the wood. The shader reads a per-shell
+`Species` attribute: rungs are pale hickory against darker oak rails, and
+end grain (the tenon ends, the rail tops) takes the finish darker. The
+rung finders now require the wood material, because the tie-rods are as
+wide as a rung. The new **tie-rod seat** budget gets its own falsifier,
+`--float-nuts` (exit 20).
+
 The joint budgets are measured per rung against each stile, from vertex
 positions in the un-raked construction frame, and the reported value is
 the worst case. Clearance is the tenon, not the barrel: the barrel is
@@ -106,7 +129,8 @@ rake toward, with its face through the rail tops, and turns the ladder
 so that side faces away from the camera.
 
 Each falsifier violates exactly one named budget. Proven on Blender
-4.5.11 LTS, 5.1.2, and 5.2.1 LTS (the binaries' own `--version`):
+4.5.11 LTS and 5.2.1 LTS (the binaries' own `--version`; 5.1.2 not re-run
+after the tie-rod pass):
 
 | Falsifier | Budget violated | Exit | Measured failure |
 | --- | --- | --- | --- |
@@ -117,8 +141,9 @@ Each falsifier violates exactly one named budget. Proven on Blender
 | `--fat-rungs` | Rung depth clearance | 17 | clearance 0 |
 | `--short-stile` | Shoe bite / cover | 18 | bite 0.070, cover −0.012 |
 | `--drift-rungs` | Rung pitch | 19 | worst gap 0.03287 off the mean |
+| `--float-nuts` | Tie-rod seat | 20 | seat bite −0.00350, thread 0.00050 proud |
 
-Default exit is 0 on all three.
+Default exit is 0 on both.
 
 ## Run
 
@@ -131,6 +156,7 @@ blender --background --python wooden_ladder.py -- --lift-z
 blender --background --python wooden_ladder.py -- --fat-rungs
 blender --background --python wooden_ladder.py -- --short-stile
 blender --background --python wooden_ladder.py -- --drift-rungs
+blender --background --python wooden_ladder.py -- --float-nuts
 blender --background --python wooden_ladder.py -- --output preview.webp --engine cycles
 ```
 
@@ -160,6 +186,7 @@ File-local. `9` is a valid check code. `10` is reserved for
 | 14 | `--output` produced no file |
 | 15 | Mesh hygiene, including coplanar pairs (`--stray-vert`, `--twin-sole`) |
 | 16 | Grounded zmin, or a rail end sitting in the tread (`--lift-z`) |
-| 17 | Part count or rung-to-stile joint fit (`--fat-rungs`) |
+| 17 | Part count or rung-to-stile joint fit, including through-tenon proud (`--fat-rungs`) |
 | 18 | Shoe bite / cover (`--short-stile`) |
 | 19 | Rung pitch (`--drift-rungs`) |
+| 20 | Tie-rod seat (`--float-nuts`) |