feat: add park-bench and wheelbarrow showcase pieces

[TMHSDigital]

Summary

Ordered showcase backlog is exhausted. Named picks: park-bench, then wheelbarrow. Next two (same spirit, restocked on ROADMAP): anvil, water-trough.

One branch, one commit per piece. Showcase 11 → 13. Did not touch examples, skills, snippets, rules, templates, gallery_framing.py, LICENSE, VERSION, CHANGELOG, or the release/smoke/pages workflows.

Composes (both): skills mesh-editing-and-bmesh, bake-high-to-low, depsgraph-and-evaluated-data, engine-export-presets; snippets bake_normal_high_to_low.py, setup_bake_target_image.py, lod_chain.py / decimate_to_budget.py, convex_hull_collider.py, export_preset_unity.py (copied, not imported).

Local binaries (version the binary reports): .scratch/blender-4.5.11-windows-x64/blender.exe 4.5.11 LTS, .scratch/blender-5.1.2-windows-x64/blender.exe 5.1.2, .scratch/blender-5.2.1-windows-x64/blender.exe 5.2.1 LTS.

park-bench (live run)

Wrought-iron slatted bench, scrolled legs, two materials. Construction closed-form (no RNG). Two 5.2.1 defaults matched.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	2000–2200	2100 / 2100 / 2100
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.4952
LOD2	0.10–0.35	0.2200 / 0.2200 / 0.1400
Materials	2, ≥48 metal	2 slots, 510 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(1.080, 0.566, 0.853) ± 0.01	(1.0800, 0.5657, 0.8531), zmin 0
Collider	≤ 140	106
Export	> 0 B	159868 / 159868 / 159852

	4.5.11	5.1.2	5.2.1
default	0	0	0
--skip-decimate	9	9	9
--nope	2	2	2

Falsifier violates LOD1 ratio (1.0 vs 0.32–0.62). Check-only wall-clock 1.3s / 1.1s / 1.3s.

wheelbarrow (live run)

Staved U-tray, iron straps, single spoked wheel (closed-form torus; spokes start at HUB_R+0.008), rear legs, handles. Two 5.2.1 defaults matched.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	2980–3200	3088 / 3088 / 3088
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2196 / 0.2196 / 0.2014
Materials	2, ≥24 metal	2 slots, 308 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(1.418, 0.463, 0.594) ± 0.01	(1.4176, 0.4627, 0.5937), zmin 0
Collider	≤ 140	106
Export	> 0 B	227404 / 227404 / 227388

	4.5.11	5.1.2	5.2.1
default	0	0	0
--skip-decimate	9	9	9
--nope	2	2	2

Falsifier violates LOD1 ratio. Check-only wall-clock 1.7s / 1.3s / 1.5s.

DECIMATE COLLAPSE is the named cross-version tolerance (ratio band). Bake pixels stochastic (FINISHED + has_data). Export bytes differ by 16 B on 5.2.1 (serializer), not gated. Heroes: Cycles 5.2.1, check_framing without deviation=. Gallery card alts are the full teaches sentence; data-tags includes showcase.

Test plan

• Default + --skip-decimate + --nope on 4.5.11, 5.1.2, 5.2.1
• Two 5.2.1 defaults identical per piece
• Fitted budgets around measured geometry (not geometry to budgets)
• Catalog + plugin.json showcase + README counts 13
• python scripts/build_gallery.py; card alts and witnesses inspected
• CI: Validate, blender-smoke 4.5 + 5.2, labeled 5.1, Socket — wait all green including 5.1 before merge