fix(showcase): grindstone and wheelbarrow visual quality pass

[TMHSDigital]

Summary

Showcase visual quality pass on grindstone and wheelbarrow — the two worst remaining after chopping-block. Structural generator fixes, then hygiene / named-support / joint-fit / seat budgets with falsifiers proven on 4.5.11, 5.1.2, and 5.2.1.

showcase/README.md now requires named supports (exit 16) on multi-foot props.

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Grindstone

Defects (inspection sheet)

1. Triangular holes under the A-frame bearings — ortho_front, ortho_back, joint_aframe_peak.
2. Angled leg end-caps piercing Z=0 / through-holes at the feet — ground_contact, ortho_bottom.
3. Coincident faces where diagonal, sill, and king shared Y-thickness (black patches) — joint_aframe_peak, clay_a.
4. Trough built from five overlapping boxes; bevel produced doubles and zero-area faces — hygiene probe.
5. Daylight between trough and sills; stretchers floating above the tie beams — ortho_top, joint_crossbrace, three_quarter_a.
6. Trough-sill audit mixed the two Y sides after crank-handle centering, so a visibly gapped trough still reported overlap.

Generator fixes (structural)

• King post + diagonals tenoning into a fatter sill/king (nested Y), not two sticks meeting at a point.
• Short wood tenon buried in the iron shoe; diagonal starts on the shoe top so the angled cap cannot pierce Z=0.
• One manifold basin (inset_region / extrude_face_region) instead of five overlapping boxes; bevel only the trough.
• TROUGH_W derived from the sill inner face plus a seat; end stretchers tenon through both sills at sill height.
• Per-side trough-sill gap; --narrow-trough / --float-legs / --short-legs as named falsifiers.
• --float-crank moves the crank inward so the Y AABB stays in band; --no-dip lowers the trough instead of raising the stone.

Budgets (all three binaries)

Axis	4.5.11 / 5.1.2 / 5.2.1
Base tris	928 / 928 / 928 (was 1604)
LOD2 ratio	0.2198 / 0.2198 / 0.1897
Hygiene	0 / 0 / 0
shoe_z, shoe_wood, trough_sill	0, 0, 0
Stone dip	0.02570
Export bytes	89316 / 89316 / 89312

Falsifiers (same code on all three)

Flag	Budget	Exit
default	—	0
--skip-decimate	LOD1	9
--stray-vert	hygiene	15
--lift-z	zmin	16
--short-legs	named shoes	16
--float-crank	crank-axle gap	17
--float-legs	shoe-wood join	17
--no-dip	stone dip band	18
--narrow-trough	trough-sill seat	18

Inspection rounds: 7. Framing on 5.2.1: fill y=0.722, margins ≥ 0.056, framing_ok.

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Wheelbarrow

Defects (inspection sheet)

1. Daylight at tray floor/wall after wood bevel — ortho_front, joint_tray_bottom.
2. Coincident faces where handle and chassis shared Y-thickness — joint_handle_tray, clay_a.
3. Shoes could float while the wheel still grounded the AABB — named-support class.
4. spoke_clearance matched floor slats, so --fat-spokes could not fail the hub budget.
5. --pipe-rim used an inner metal torus that floated (tyre_z=0.008) and hit exit 16 before tread aspect.
6. --float-walls at a zero seat welded four doubles, so hygiene 15 fired before seat 17.

Generator fixes (structural)

• Negative slat gap and walls seated into the floor (WALL_SEAT).
• Handle-end thinner in Y so it tenons into a fatter chassis.
• Spoke metric is hub-local (centroid near the wheel), not "thin-in-Z".
• Pipe-rim falsifier: metal is the outer torus, planted from its own outer radius at Z=0.
• --float-walls uses a small negative seat so walls lift without kissing/welding.

Budgets (all three binaries)

Axis	4.5.11 / 5.1.2 / 5.2.1
Base tris	2500 / 2500 / 2500 (was 3088)
LOD2 ratio	0.2200 / 0.2200 / 0.2104
Hygiene	0 / 0 / 0
shoe_z, tyre_z, wall-floor seat	0, 0, 0.012
Spoke clearance / tread aspect	0.040 / 5.111
Export bytes	187576 / 187576 / 187560

Falsifiers (same code on all three)

Flag	Budget	Exit
default	—	0
--skip-decimate	LOD1	9
--stray-vert	hygiene	15
--lift-z	zmin	16
--short-legs	named shoes	16
--fat-spokes	spoke clearance	17
--float-walls	wall-floor seat	17
--pipe-rim	tread aspect	18

Inspection rounds: 6. Framing on 5.2.1: fill y=0.728, margins ≥ 0.067, framing_ok.

Test plan

• Default exit 0 on 4.5.11, 5.1.2, 5.2.1 for both pieces
• Every named falsifier fires the documented code on all three
• Hero stills on 5.2.1, check_framing without deviation=, framing_ok
• Generated gallery HTML alts and witnesses callouts read in full (not truncated)
• CI blender-smoke 5.2 + 4.5 + labeled 5.1