fix(showcase): remodel mushroom-stump with a stepped felling cut, recessed peels, conforming moss and bipinnate ferns

The stump read oddly in review: the torn holding wood stood up in the
middle of a level cut as a 13 cm comb of splinters, the peeled patches
looked like planks pasted on the trunk, the moss was a flat green slab,
the ferns were the cartoon fronds of fallen-log, and the stump sat small
on a wide soil disc. It is rebuilt:

- the felling: the back cut is level; beyond the hinge chord (0.172 m
  toward the fall, now to the camera's right) the undercut floor lies a
  34 mm step lower. The top is bisected along both edges of the step face
  so the step is its own strip of faces, and a low band of torn hinge
  fibres (up to 29 mm, irregular tips, leaning toward the fall) runs along
  the step's edge over the middle two thirds of the chord, its front face
  running down the step to below the undercut floor. Past its ends the
  step is a clean sawn face
- peels: the sapwood is the trunk's own surface 26 mm under the plate
  crests, weathered grey-brown with vertical fibre streaks, fine checks
  and bark-beetle galleries; the torn bark curls up to 6 mm proud in a
  ragged lip showing rust-brown inner bark; 12 bark flakes lift off the
  edges, roots set in the bark, undersides inner bark
- moss: four thin cushions (north flank, a small one high, two root
  crotches) laid on the bark as built, furrows and all, ~10 mm thick,
  thinning to a lobed rim tucked 2 mm under the bark with tongues along
  the furrows, 19 satellite tufts, the rim colour thinning via Zone
- ferns: twelve bipinnate fronds (0.40-0.64 m) in two clumps either side
  of the stump, the frond builder of fern-mossy-rock copied inline at
  about half size (alternate pinnae, pinnules, pinna separation, sori)
- composition: soil disc 2.0x1.9 -> 1.7x1.6 m, roots, agarics, twigs,
  pebbles and fern clumps pulled in; camera 3.7 -> 3.3 m; lathe 96 -> 128
  sides; bark cross-cracks and end-grain glare toned down; key/wedge
  relit so warmth sits inside the calibration spread
- coplanar hygiene fixed by construction, not bands: moss bases belly
  into the stump by their own depth, tuft bases are cones, honey caps tip
  a hashed way each, pinnae separated as in fern-mossy-rock

Budget changes:
- Base tris 34700-35800 -> 56150-57250 (measured 56710; LOD 0.5000/0.2200)
- Outer AABB -> (1.9351, 1.6283, 0.6888) m; face floors re-fit
  (4441/1702/3080/1596/672/1820/4050/1758/1232/17884/384/210 measured)
- Cut level now fit on the back cut only (Notch attr): 0.0894 deg;
  size 0.6210 / 0.4966 m
- New 25 felling: undercut step 0.028-0.041 (0.0346), hinge tallest fibre
  0.018-0.034 (0.0286)
- New 26 peel: torn bark over sapwood 0.020-0.034 (0.0268, 0.0249), 12
  flakes rooted 0.002-0.020 (0.0042-0.0150)
- New 27 moss conforms: greatest height over bark 0.006-0.016 (0.0104)
- Asset-quality remap 25 -> 28

Falsifiers (flag -> measured -> exit), all on the final code:
  --skip-decimate    LOD1 1.0000                          9
  --stray-vert       loose_v 1                           15
  --lift-z           zmin 0.05000                        16
  --perch-stump      sealed 6/8                          17
  --float-caps       bite -0.0156..-0.0100               18
  --tilt-cut         4.9584 deg                          19
  --fat-stump        0.6706 m across                     19
  --arch-roots       -0.0524 (lift 90 -> 120 mm)         20
  --float-mushrooms  -0.0249..-0.0032                    21
  --float-brackets   -0.0164..-0.0131                    22
  --sunny-moss       0.4270                              23
  --float-cover      77/339 loose                        24
  --flat-felling     step 0.0006                         25
  --tall-hinge       tip 0.0358 (top +7.2 mm, tol 10)    25
  --flush-peel       -0.0015, -0.0011                    26
  --float-flakes     -0.0108..0.0011                     26
  --slab-moss        0.0360                              27
Every falsifier keeps 56710 tris and the default AABB except --tall-hinge
(tallest fibre sets the top; +7.2 mm inside BBOX_TOL). The lip's own stand
over the bark was tried as a sub-budget and dropped: its median read the
flare and lobes, not the lip (flush and default measured alike).

Evidence (live runs, local):
- .scratch/5.2/blender-5.2.1-windows-x64/blender.exe (5.2.1 LTS): two
  default runs identical; all 17 falsifiers exit their codes;
  tests/check_falsifier_targets.py --run --only mushroom-stump passes.
- .scratch/blender-4.5.11-windows-x64/blender.exe (4.5.11 LTS): same
  sweep, same codes, identical measured lines (glTF +16 bytes).
- .scratch/blender-5.1.2-windows-x64/blender.exe (5.1.2): default exit 0,
  identical measured lines.
- Framing: fill 0.709 x 0.833; margins l .122 r .169 b .039 t .128.
- Contact sheet: luma 0.2391 (band 0.2369-0.2428), stage 0.2534,
  warmth +0.0994 (spread -0.144..+0.105; was +0.1456), sat 0.1565.
- Asset sheet: the stump now carries a readable felling step, recessed
  peels and bipinnate ferns; not the least-designed object in the lineup.

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

Author: TMHSDigital

README.md

@@ -170,7 +170,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
   <a href="showcase/wingback-armchair/"><img src="showcase/wingback-armchair/preview.webp" width="24%" alt="Wingback armchair: an oxblood leather wingback with a deep-button tufted back, rolled arms trimmed with brass nailheads and a piped seat cushion, beside a side table with a brass reading lamp and books on a fringed rug" /></a>
   <a href="showcase/planet-rover/"><img src="showcase/planet-rover/preview.webp" width="24%" alt="Planet rover: a six-wheeled rocker-bogie rover with gold-foil sides, a camera mast, a dish antenna and a finned power unit, its arm drilling a boulder on red-grey regolith" /></a>
   <a href="showcase/traffic-cones/"><img src="showcase/traffic-cones/preview.webp" width="24%" alt="Traffic cones: orange cones with white reflective collars on a patch of asphalt with a worn white line, three standing in a row, three nested in a stack and one knocked over showing its open base, before a striped A-frame barricade with an amber warning lamp" /></a>
-  <a href="showcase/mushroom-stump/"><img src="showcase/mushroom-stump/preview.webp" width="24%" alt="Mushroom stump: a sawn tree stump with ringed, cracked end grain and torn splinters on a sloping soil disc, buttress roots in the soil, moss, banded shelf fungi, honey mushroom clusters, red fly agarics, ferns and fallen leaves" /></a>
+  <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: yellow go-kart on a patch of track asphalt before a red and white kerb: bent red tube chassis, black seat, finned engine with chain to the rear axle, slick tyres on silver rims." /></a>
   <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>
@@ -296,7 +296,7 @@ Budget-conformance props. Not examples. Conventions: [`showcase/README.md`](show
 
 [`traffic-cones`](showcase/traffic-cones/) — the seventh `vehicles` piece: procedural roadworks set on a cut-out patch of asphalt (a broken edge, a worn white road line and two sealed tar cracks; seven 720 mm PVC cones, each a hollow lathed body with a moulded bead and a rolled lip round an open tip, bonded into a square rubber base with chamfered corners, a raked top, a collar, four lugs and a recess underneath, and hooped with two retroreflective collars; three standing in a taper line, three nested in a stack and one knocked over onto the edge of its base and its lip; a folding A-frame barricade of four square steel legs hinged in pairs on pivot bolts, spread by pinned straps, footed in rubber pads, with two striped boards bolted to each face and a warning lamp with amber fresnel lenses on a bracket), the cone wall sized so each cone in the stack bears 1 mm on the one below at a 36 mm pitch. `--float-cone` exits 16 on every support bedded in the slab, `--gap-board` 17 on the boards biting their legs, `--float-band` 18 on the collars seated on their bodies, `--lean-cone` and `--skew-leg` 19 on the cones plumb at real size and the legs mirror-symmetric, `--loose-stack` 20 on the nesting pitch and wall bite, `--loose-lamp` 21 on one connected assembly, while the envelope holds.
 
-[`mushroom-stump`](showcase/mushroom-stump/) — the seventh `nature` piece: procedural felled-tree stump on a sloping forest-floor soil disc (one furrowed lathe about a plumb axis flaring through five buttresses into the ground, plated bark peeled to sapwood in two patches, a level saw cut with weathered growth rings round a dark heartwood, radial drying checks notching the rim and a strip of torn holding wood with long splinters, five surface roots lying in the soil and diving into it, moss on the shaded flank, two tiers of turkey-tail brackets, three fly agarics with warted caps, pleated gills, a volva and a hanging skirt, two clusters of ringed honey fungus out of a root and the stump's foot, fallen leaves, ferns, twigs and pebbles), whose foot sealed in the soil all round, caps seated on their stipes, level cut and stated size, roots bedded along their run, stipes rooted in their soil or wood, brackets rooted in the bark, moss on up- and shade-facing surfaces and ground cover joined to the soil are recomputed from the mesh, through the same pipeline. `--perch-stump` exits 17 on the stump sealed all round, `--float-caps` 18 on every cap seated on its stipe, `--tilt-cut` and `--fat-stump` 19 on the level cut and stated size, `--arch-roots` 20 on the roots bedded, `--float-mushrooms` 21 on every stipe rooted in its host, `--float-brackets` 22 on the brackets rooted in the bark, `--sunny-moss` 23 on moss facing up or into the shade, `--float-cover` 24 on the ground cover joined to the soil, while the envelope holds.
+[`mushroom-stump`](showcase/mushroom-stump/) — the seventh `nature` piece: procedural felled-tree stump on a sloping forest-floor soil disc (one furrowed lathe about a plumb axis flaring through five buttresses into the ground, plated bark peeled in two patches to grey sapwood under a ragged torn lip with curling flakes, a felling top with weathered growth rings round a dark heartwood, radial drying checks notching the rim, the undercut floor a step under the back cut and a low band of torn hinge fibres along the step, five surface roots lying in the soil and diving into it, thin moss cushions laid on the furrowed bark on the shaded flank and in the root crotches, two tiers of turkey-tail brackets, three fly agarics with warted caps, pleated gills, a volva and a hanging skirt, two clusters of ringed honey fungus out of a root and the stump's foot, fallen leaves, bipinnate ferns, twigs and pebbles), whose foot sealed in the soil all round, caps seated on their stipes, level cut and stated size, roots bedded along their run, stipes rooted in their soil or wood, brackets rooted in the bark, moss on up- and shade-facing surfaces, ground cover joined to the soil, the felling step and low hinge band, the torn bark standing over the sapwood with its flakes rooted, and the moss a thin shell on the bark are recomputed from the mesh, through the same pipeline. `--perch-stump` exits 17 on the stump sealed all round, `--float-caps` 18 on every cap seated on its stipe, `--tilt-cut` and `--fat-stump` 19 on the level cut and stated size, `--arch-roots` 20 on the roots bedded, `--float-mushrooms` 21 on every stipe rooted in its host, `--float-brackets` 22 on the brackets rooted in the bark, `--sunny-moss` 23 on moss facing up or into the shade, `--float-cover` 24 on the ground cover joined to the soil, `--flat-felling` and `--tall-hinge` 25 on the felling step and low hinge band, `--flush-peel` and `--float-flakes` 26 on the torn bark over the sapwood and its flakes rooted, `--slab-moss` 27 on the moss laid on the bark, while the envelope holds.
 
 [`go-kart`](showcase/go-kart/) — the eighth `vehicles` piece: procedural rental go-kart on a patch of kart-track asphalt against a red and white kerb (a 30 mm tube chassis whose main loop is one bar swept round every bend, cross tubes ending inside the rails with welded gussets, bearing hangers and kingpin C-brackets; spindles on kingpin bolts with 14° caster; a three-spoke wheel on a raked column in a bushing, a steering plate and two tie rods on rod-end eyes; a bolted nose, side pods on nerf bars and a curved front panel; a moulded seat on stays, a floor tray, two pedals, a strapped fuel tank with its line and a lead ballast block; a finned four-stroke clamped to the right rail with a pull-start, air-box and silencer, driving the live rear axle through a clutch, an 88-link chain solved to a whole number of pitches and a 60-tooth sprocket; bearing carriers, a brake disc and caliper; a tubular rear bumper; four loaded slicks on split rims with valve stems, the fronts narrower than the rears), whose tyres pressed into the asphalt in one plane, hubs and bearings coaxial with their axles, tie-rod eyes on their pins, tyre beads and chain seated, frame, bodywork and wheels mirrored at the stated wheelbase and tracks, caster, mass centre inside the support polygon and one connected assembly are recomputed from the mesh, through the same pipeline. `--float-tyre` exits 16 on every tyre grounded, `--cock-hub`, `--drop-bearing` and `--short-tierod` 17 on joint fit, `--sink-tyre` and `--lift-chain` 18 on the bead and chain seats, `--toe-wheel`, `--wide-track`, `--odd-frame` and `--no-caster` 19 on mirror, size and caster, `--aft-ballast` 20 on the stance, `--loose-ballast` 21 on one connected assembly, while the envelope holds.