fix(showcase): wrap farm-tractor's chevron bars round the shoulder, 34 mm tall, with mud packed between them

The rear tyres' V-bar lugs, the tractor's signature, barely read in the
hero: the bars stopped at the shoulder at 28 mm on a near-black carcass,
so at the hero's side-on angle the tread looked almost smooth and the
sidewall was a blank black disc. Rebuilt:

- carcass crown lowered 6 mm (0.592 -> 0.586 m) and the bars raised
  28 -> 34 mm, so the 1.240 m diameter over the lugs is unchanged; bars
  narrowed (top 32 -> 26 mm, root 46 -> 42 mm) for a sharper, more open
  tread
- each bar now runs past its sweep round the shoulder and 21 mm down the
  sidewall (to r = 0.518 m), every section set out along the carcass
  normal rather than the radius, tapering to 55 % of its height at the
  end: the bars' ends stand as a ring of teeth round the tyre seen from
  the side, as on a real R1 tyre. 9 stations per bar (was 7), placed on
  the carcass profile's vertices so the root follows it exactly
- REAR_CROWN is now the single source of the tread profile: the carcass
  lathe and the bars both read it
- a TreadMud face attribute marks the rear carcasses' crowns; the rubber
  packs them with clumped, lumpy mud there, so the clean black bars stand
  out of it (the far tyre's crown shows it in the hero)

Budget changes (5.2.1):
- Base tris 121000-123600 -> 123100-125700 (124384; LOD 0.5000/0.2198)
- Rubber face floor 14290 -> 15260 (16592 measured, was 15536)
- Lugs: roots 3.932-3.997 -> 3.985-3.999 mm (band 2.0-6.0 kept), proud
  28.311 -> 34.289 mm, floor >= 20 -> >= 30 mm. Depth is now read by the
  nearest carcass face (inside or out still by the radial ray): down the
  shoulder the carcass normal runs nearly along the axle, and a radial
  depth there read 17 mm for a 4 mm bite
- Stance 0.4151 -> 0.4120 m (>= 0.330; 1539 kg)
- Unchanged: 12 materials, AABB (3.5839, 2.0869, 1.7330), collider 944,
  hygiene 0 incl. 0 coplanar pairs, 4 tyres sunk 28.9-30.1 mm, seats
  0.683-0.962 mm, pitch 0.0000 deg, handing 10.886 deg, mirrors 0.0001 mm,
  one assembly of 505 shells

Falsifiers (flag -> measured -> exit), 5.2.1 and 4.5.11 identical; tris
124384 and envelope (3.5839, 2.0869, 1.7330) unchanged in all:
  --skip-decimate  LOD1 1.0000                                   9
  --stray-vert     loose_v 1                                    15
  --lift-z         zmin 0.05000                                 16
  --float-tyre     left front sink -9.889 mm                    16
  --cock-hub       left rear hub 1.5 mm off its half-shaft      17
  --cant-pin       bushings 1.894 mm, 1.5 deg off the pin       17
  --unpin-link     top-link rear eye 8.0 mm off its pin         17
  --sink-tyre      right rear bead seat 2.762 mm                18
  --float-lugs     left rear bar roots -3.056 mm (was -3.068)   18
  --skew-wheel     wheel mirror 6.000 mm                        19
  --wide-track     rear track 1.3280 m                          19
  --tall-lugs      rear tyres 1.256 m                           19
  --odd-fender     body mirror 3.000 mm                         19
  --bunch-lugs     bar pitch off 2.000 deg                      20
  --reverse-lugs   bar handing -10.886 deg                      21
  --offset-pivot   stance margin 0.3100 m (was 0.3074)          22
  --loose-lamp     2 components [4, 501]                        23
--float-lugs now lifts the bars 7 mm along the carcass normal: a radial
lift would have left the wrapped ends inside the steep sidewall.
tests/check_falsifier_targets.py --run --only farm-tractor passes.

Blender: E:\Blender-Developer-Tools\.scratch\5.2\blender-5.2.1-windows-x64\blender.exe
("Blender 5.2.1 LTS"): default and every falsifier;
.scratch\blender-4.5.11-windows-x64 ("Blender 4.5.11 LTS"): default and
every falsifier, same measurements (LOD2 27364 vs 27340 tris, ratio
0.2200 vs 0.2198); .scratch\blender-5.1.2-windows-x64 ("Blender 5.1.2"):
two defaults, identical, same measurements as 4.5.11.

Gates (5.2.1 renders):
- Framing: fill x 0.712, y 0.861; margins l .144 r .144 b .050 t .089
- Asset quality: 12 materials, edge90 0.048, no default names
- Contact sheet (docs/gallery/contact-sheets/farm-tractor-contact-sheet.webp):
  luma 0.2407 (band 0.2369-0.2428), stage 0.2303 (<= 0.2517), warmth
  +0.0464 (-0.144..+0.105), saturation 0.1639 (0.1525-0.2367); thumbnail
  legible, the lugs now read at thumbnail size
- Asset sheet (docs/gallery/asset-sheets/farm-tractor.webp): before, the
  rear tyres read as smooth black discs; after, both rear tyres carry a
  ring of bar ends and the chevrons read across the top of the far tyre.
  Not the least-designed object in the lineup. Weak point kept: the mud
  patch still makes the tractor smaller in its panel
- build_gallery, check_site_links (122 pages, 0 failures) and every
  tests/check_*.py pass; generated alt read in docs/gallery/index.html

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

Author: TMHSDigital

README.md

@@ -173,7 +173,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
   <a href="showcase/mushroom-stump/"><img src="showcase/mushroom-stump/preview.webp" width="24%" alt="Mushroom stump: a sawn tree stump with a stepped felling cut and torn hinge fibres, peeled bark with curling flakes, moss, banded shelf fungi, honey mushroom clusters, red fly agarics, ferns and fallen leaves on a sloping soil disc" /></a>
   <a href="showcase/go-kart/"><img src="showcase/go-kart/preview.webp" width="24%" alt="Go-kart: blue go-kart on a patch of track asphalt before a red and white kerb: moulded nose, side pods and a front panel with a white plate, red tube chassis, black seat, finned engine." /></a>
   <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/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 rear tyres with mud-packed chevron lugs 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 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>
@@ -302,7 +302,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
 
 [`broadleaf-oak`](showcase/broadleaf-oak/) — the eighth `nature` piece: procedural mature, open-grown English oak on a grassy mound (a massive, short trunk with broad flutes and deep-furrowed bark plates flaring through seven buttresses into surface roots bedded in the soil, a low fork into five zig-zag scaffold limbs, the low ones reaching out sideways nearly level, that divide three more times into 155 tapered branches, each seated in its parent and narrowing past every junction by its child's area, a domed crown of 295 leaf-mass cushions — a hidden core on each carrier, 2075 faceted leaf clusters centred in the cores and shaded as billows by custom normals, 618 small lobed oak leaves on the outer clusters — dead twigs, acorns with scaly cups and glossy nuts hanging on stalks and lying in the grass among leaf litter, forked twigs and tufts), whose branch seats, da Vinci taper, nuts seated in their cups, plumb trunk and balanced crown, foot and roots sealed in the soil, cushions on their carriers, clusters in their cores and leaves in their clusters, acorns hung on their stalks and ground cover joined to the soil are recomputed from the mesh, through the same pipeline. `--float-branches` exits 17 on every branch seated in its parent, `--pop-nuts` 18 on every nut seated in its cup, `--lean-crown` 19 on plumb and balance, `--fat-twigs` 20 on the taper and area rule, `--perch-trunk` and `--arch-roots` 21 on the trunk and roots sealed in the soil, `--orphan-clump`, `--scatter-clusters` and `--shed-leaves` 22 on every cushion on its carrier, cluster in its core and leaf in its cluster, `--drop-acorns` 23 on the hanging acorns on their stalks, `--float-cover` 24 on the ground cover joined to the soil, while the envelope holds.
 
-[`farm-tractor`](showcase/farm-tractor/) — the ninth `vehicles` piece: procedural mid-century utility tractor standing in the ruts of a muddy farmyard with a puddle between its wheels (a cast-iron backbone of engine, bell housing, gearbox and rear axle housing with trumpets; a front axle oscillating on a pivot pin through the front support's lugs and two bushings, kingpins, spindles and a tie rod; 1.240 m rear tyres with 44 chevron bars rooted in the carcass and handed for forward travel, on dished rims with welded clamp lugs and cast-iron half-moon weights; ribbed front tyres; a grille shell with radiator and headlamps, a louvred bonnet, fuel tank and dash; a vertical exhaust stack with a hinged rain cap and an oil-bath pre-cleaner bowl; a raked steering column, a pan seat on a leaf spring, pedals and footplates; mudguards with a tail lamp and a work lamp; a three-point linkage with lift arms, lift rods, lower links, a stowed top link and the drawbar, and a PTO stub under its shield; paint chipped to primer at its edges, rust-streaked, sun-faded and mud-splashed). Asserts 122272 tris, twelve materials with face floors, zero coplanar cross-shell pairs, every tyre sunk 18–45 mm into the mud, hubs and the pivot pin coaxial, 14 link eyes on their pins, beads seated, 88 bars rooted and evenly pitched with their apex leading, body and wheels mirrored, wheelbase, tracks and rear-tyre diameter at their stated size, the mass centre inside the support triangle of the rear contacts and the front pivot by a quarter of the track, and one connected assembly; `--float-tyre` exits 16, `--cock-hub`, `--cant-pin` and `--unpin-link` 17, `--sink-tyre` and `--float-lugs` 18, `--skew-wheel`, `--wide-track`, `--tall-lugs` and `--odd-fender` 19, `--bunch-lugs` 20, `--reverse-lugs` 21, `--offset-pivot` 22, `--loose-lamp` 23, while the envelope holds.
+[`farm-tractor`](showcase/farm-tractor/) — the ninth `vehicles` piece: procedural mid-century utility tractor standing in the ruts of a muddy farmyard with a puddle between its wheels (a cast-iron backbone of engine, bell housing, gearbox and rear axle housing with trumpets; a front axle oscillating on a pivot pin through the front support's lugs and two bushings, kingpins, spindles and a tie rod; 1.240 m rear tyres with 44 chevron bars rooted in the carcass, handed for forward travel and wrapped round the shoulder, mud packed between them, on dished rims with welded clamp lugs and cast-iron half-moon weights; ribbed front tyres; a grille shell with radiator and headlamps, a louvred bonnet, fuel tank and dash; a vertical exhaust stack with a hinged rain cap and an oil-bath pre-cleaner bowl; a raked steering column, a pan seat on a leaf spring, pedals and footplates; mudguards with a tail lamp and a work lamp; a three-point linkage with lift arms, lift rods, lower links, a stowed top link and the drawbar, and a PTO stub under its shield; paint chipped to primer at its edges, rust-streaked, sun-faded and mud-splashed). Asserts 124384 tris, twelve materials with face floors, zero coplanar cross-shell pairs, every tyre sunk 18–45 mm into the mud, hubs and the pivot pin coaxial, 14 link eyes on their pins, beads seated, 88 bars rooted and evenly pitched with their apex leading, body and wheels mirrored, wheelbase, tracks and rear-tyre diameter at their stated size, the mass centre inside the support triangle of the rear contacts and the front pivot by a quarter of the track, and one connected assembly; `--float-tyre` exits 16, `--cock-hub`, `--cant-pin` and `--unpin-link` 17, `--sink-tyre` and `--float-lugs` 18, `--skew-wheel`, `--wide-track`, `--tall-lugs` and `--odd-fender` 19, `--bunch-lugs` 20, `--reverse-lugs` 21, `--offset-pivot` 22, `--loose-lamp` 23, while the envelope holds.
 
 [`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.