fix(showcase): turn wooden-ladder rungs and give its wood readable grain

Inside the exact 16-shell parts budget: each rung gains one mid-span ring,
so the barrel shoulders in at 0.90x (RUNG_SHOULDER) and swells to 1.14x
(RUNG_SWELL) - a lathe-turned round instead of a dowel. Tenons unchanged,
so clearance/engagement/pitch read the same; breakout 0.03780 -> 0.03787.
Wood shader: grain scale 34 -> 95 and streak stretch 0.94 -> 0.985 so
streaks run across a 50 mm stile (it rendered flat tan), plus a short-range
AO grime term at the rung housings and under the iron sleeves.

Evidence (live, local):
- Blender 5.2.1 LTS (.scratch): default exit 0, base_tris 2680 (budget
  1800-2800), r1 0.5000 r2 0.2194, parts 16, wood 684 / metal 736 faces,
  bbox unchanged, collider 24; --output framing fill y=0.883.
- Blender 4.5.11 LTS (.scratch): default exit 0, base_tris 2680, r2 0.2194.
- check_falsifier_targets.py --run, both binaries: all wooden-ladder
  falsifiers return their codes (9, 15, 15, 16, 17, 18, 19).
- Contact sheet: luma 0.211 / stage 0.202 / warmth +0.089 / sat 0.172.
- Asset sheet: still the plainest object in the lineup at sheet scale; the
  turned rungs read up close, not at thumbnail size. Honest verdict: improved,
  not cleared.

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

Author: TMHSDigital

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

@@ -1,8 +1,9 @@
 # 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,
-barrel thicker only in the clear span), iron ferrules at the top, and
+tapered from 64 mm to 50 mm, raked 12°, six turned rungs (22 mm tenon, 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
 pipeline: unique-cell UVs, Cycles high-to-low normal bake, LOD chain,
 convex collider, Unity glTF export.
@@ -36,30 +37,44 @@ 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 / 5.1.2 / 5.2.1) |
+| Axis | Declared | Measured (4.5.11 / 5.2.1; 5.1.2 not re-run after the turned-rung pass) |
 | --- | --- | --- |
-| Base triangles | 1800–2800 | 2536 / 2536 / 2536 |
-| LOD1 ratio | 0.32–0.62 of base | 0.5000 / 0.5000 / 0.5000 |
-| LOD2 ratio | 0.10–0.35 of base | 0.2192 / 0.2192 / 0.2192 |
-| Materials | exactly 2 distinct, ≥280 wood, ≥180 metal | 2 slots, 612 wood, 736 metal |
+| Base triangles | 1800–2800 | 2680 / 2680 |
+| 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 |
 | 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) |
 | 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) |
 | 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 | 0.03780 |
+| Tenon breakout margin | ≥ 0.008 m short of the stile outer face | 0.03787 |
 | 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 | 187524 / 187524 / 187504 bytes |
+| Export | written, size > 0 | 197604 / 197584 bytes |
 
 Base triangles rose from **936 to 2536**. The previous mesh was two
 boxes, six cloned dowels, and four chrome cubes. Turned rungs (tenon
 and barrel in one shell) plus ferrule collars and chamfered soles added
 the triangles. Nothing was removed as a hidden face.
 
+A later pass took them to **2680**. The bare ladder was still the plainest
+object in its asset-sheet lineup, and the parts budget is exact (16
+shells), so the detail had to live inside existing shells. Each rung gained
+one ring at mid-span (+24 triangles per rung): the barrel shoulders in at
+0.90 × its radius (`RUNG_SHOULDER`) and swells to 1.14 × (`RUNG_SWELL`).
+That turns a dowel into a lathe-turned round. The tenons are unchanged, so
+clearance, engagement and pitch read the same. The breakout margin moved
+0.07 mm (0.03780 → 0.03787) because the shell's worst vertex is now measured
+across seven rings. The wood shader also changed. The grain noise runs
+across the stile's 50 mm width at a readable scale (`WOOD_GRAIN_SCALE`
+34 → 95, streak stretch 0.94 → 0.985); before, about one noise period
+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.
+
 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

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

@@ -65,6 +65,8 @@
 BARREL_RATIO = 1.34
 RUNG_SEGS = 12
 RUNG_TENON = 0.014
+RUNG_SHOULDER = 0.90             # barrel radius at the shoulders, x r_barrel
+RUNG_SWELL = 1.14                # barrel radius at mid-span, x r_barrel
 # Rungs sit at one even pitch: a climber's feet find them blind, so height
 # jitter is a defect, not variation. Variation lives in the turning (a few
 # percent of barrel) and the wood tone. --drift-rungs restores the old
@@ -75,7 +77,7 @@
 # Per-piece wood tone jitter and grain frequency, as in shipping-crate.
 PLANK_TONE_JITTER = 0.28
 TONE_SEED = 29
-WOOD_GRAIN_SCALE = 34.0
+WOOD_GRAIN_SCALE = 95.0
 BAND_Z0 = 0.0
 BAND_H = 0.058
 BAND_T = 0.007
@@ -269,16 +271,23 @@ def quad(a, b, c, d):
 
 
 def add_turned_rung(bm, z, x_left, x_right, tenon, r_tenon, r_barrel, segs, mat_idx):
-    """One shell: thin tenon inside each stile, thicker barrel only in the clear span."""
+    """One shell: thin tenon inside each stile, thicker barrel only in the clear span.
+
+    The barrel is turned, not a plain dowel: it starts at a shoulder just
+    under the barrel radius and swells to RUNG_SWELL at mid-span, the
+    profile a lathe leaves on a ladder round (one extra ring per rung).
+    """
     xs = (
         x_left - tenon,
         x_left - 0.0015,
         x_left + 0.004,
+        0.5 * (x_left + x_right),
         x_right - 0.004,
         x_right + 0.0015,
         x_right + tenon,
     )
-    rs = (r_tenon, r_tenon, r_barrel, r_barrel, r_tenon, r_tenon)
+    shoulder = r_barrel * RUNG_SHOULDER
+    rs = (r_tenon, r_tenon, shoulder, r_barrel * RUNG_SWELL, shoulder, r_tenon, r_tenon)
     rings = []
     for x, radius in zip(xs, rs):
         ring = []
@@ -969,7 +978,7 @@ def wood_material(name):
     nt.links.new(gdir.outputs["Vector"], dot.inputs[1])
     squash = nt.nodes.new("ShaderNodeMath")
     squash.operation = "MULTIPLY"
-    squash.inputs[1].default_value = 0.94
+    squash.inputs[1].default_value = 0.985
     nt.links.new(dot.outputs["Value"], squash.inputs[0])
     along = nt.nodes.new("ShaderNodeVectorMath")
     along.operation = "SCALE"
@@ -989,10 +998,10 @@ def wood_material(name):
     noise.inputs["Roughness"].default_value = 0.62
     nt.links.new(shift.outputs["Vector"], noise.inputs["Vector"])
     ramp = nt.nodes.new("ShaderNodeValToRGB")
-    ramp.color_ramp.elements[0].position = 0.30
-    ramp.color_ramp.elements[0].color = (0.10, 0.056, 0.028, 1.0)
-    ramp.color_ramp.elements[1].position = 0.72
-    ramp.color_ramp.elements[1].color = (0.33, 0.19, 0.095, 1.0)
+    ramp.color_ramp.elements[0].position = 0.38
+    ramp.color_ramp.elements[0].color = (0.055, 0.030, 0.014, 1.0)
+    ramp.color_ramp.elements[1].position = 0.62
+    ramp.color_ramp.elements[1].color = (0.34, 0.20, 0.10, 1.0)
     nt.links.new(noise.outputs["Fac"], ramp.inputs["Fac"])
     gain = nt.nodes.new("ShaderNodeMath")
     gain.operation = "MULTIPLY_ADD"
@@ -1005,7 +1014,21 @@ def wood_material(name):
     _sock(mix.inputs, "Factor_Float").default_value = 1.0
     nt.links.new(ramp.outputs["Color"], _sock(mix.inputs, "A_Color"))
     nt.links.new(gain.outputs["Value"], _sock(mix.inputs, "B_Color"))
-    nt.links.new(_sock(mix.outputs, "Result_Color"), bsdf.inputs["Base Color"])
+    # handling grime: short-range AO darkens where rungs enter the stiles
+    # and under the iron sleeves, so the joints read at thumbnail scale
+    ao = nt.nodes.new("ShaderNodeAmbientOcclusion")
+    ao.inputs["Distance"].default_value = 0.035
+    grime = nt.nodes.new("ShaderNodeMapRange")
+    grime.inputs["To Min"].default_value = 0.45
+    grime.inputs["To Max"].default_value = 1.0
+    nt.links.new(ao.outputs["AO"], grime.inputs["Value"])
+    dirt = nt.nodes.new("ShaderNodeMix")
+    dirt.data_type = "RGBA"
+    dirt.blend_type = "MULTIPLY"
+    _sock(dirt.inputs, "Factor_Float").default_value = 1.0
+    nt.links.new(_sock(mix.outputs, "Result_Color"), _sock(dirt.inputs, "A_Color"))
+    nt.links.new(grime.outputs["Result"], _sock(dirt.inputs, "B_Color"))
+    nt.links.new(_sock(dirt.outputs, "Result_Color"), bsdf.inputs["Base Color"])
     rough = nt.nodes.new("ShaderNodeMapRange")
     rough.inputs["To Min"].default_value = 0.72
     rough.inputs["To Max"].default_value = 0.52