feat: add bellows and chopping-block showcase pieces

[TMHSDigital]

Summary

Numbered showcase backlog is exhausted. Next two ROADMAP candidates, now shipped:

• showcase/bellows/ — hinged timber paddles, leather bag, iron nozzle
• showcase/chopping-block/ — hooped timber stump with an embedded axe

Picks (this loop): blacksmith bellows and chopping block with embedded axe. Restocked ROADMAP with wooden bucket + wall torch sconce.

Showcase count 19 → 21. One feat: commit per piece. Did not touch examples, skills, snippets, rules, templates, gallery_framing.py, LICENSE, VERSION, CHANGELOG, release.yml, blender-smoke.yml, or pages.yml.

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).

Falsifier (both, all three binaries): --skip-decimate → exit 9 on the LOD1 ratio budget. Argparse --nope → 2.

Bellows — live run (.scratch binaries)

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	1180–1380	1272 / 1272 / 1272
LOD1 / LOD2 ratio	0.32–0.62 / 0.10–0.35	0.5000 / 0.5000 / 0.5000 and 0.2186 / 0.2186 / 0.2170
Materials	3 distinct, ≥24 leather + metal	294 leather, 48 metal
UVs	0..1, overlap ≤ 1e-5	in range, overlap 0
AABB	(0.630, 0.403, 0.277) ± 0.01	(0.6303, 0.4031, 0.2765), zmin 0
Collider	≤ 180	148
Export	size > 0	105796 / 105796 / 105788 B

Exits: default 0 / --skip-decimate 9 / --nope 2 on 4.5.11, 5.1.2, 5.2.1. Two identical 5.2.1 defaults (1272 tris, same AABB). 5.2.1 LOD2 is 2 tris leaner (ratio band, named in README). Smoke wall-clock: 0.98s / 0.97s / 1.07s.

Chopping-block — live run

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	1880–2160	2016 / 2016 / 2016
LOD1 / LOD2 ratio	0.32–0.62 / 0.10–0.35	0.5000 / 0.5000 / 0.5000 and 0.2192 / 0.2192 / 0.2183
Materials	2 distinct, ≥24 metal	122 metal, 842 wood
UVs	0..1, overlap ≤ 1e-5	in range, overlap 0
AABB	(0.471, 0.463, 0.841) ± 0.01	(0.4713, 0.4626, 0.8413), zmin 0
Collider	≤ 220	188
Export	size > 0	157000 / 157000 / 156992 B

Exits: default 0 / --skip-decimate 9 / --nope 2 on all three. Two identical 5.2.1 defaults. Smoke wall-clock: 1.16s / 0.98s / 1.16s.

Test plan

• Default 0, --skip-decimate 9, --nope 2 on 4.5.11, 5.1.2, 5.2.1
• Dual 5.2.1 defaults match (tris + AABB)
• Heroes Cycles 5.2.1; Pillow webp q=85 method=6; check_framing without deviation=
• plugin.json showcase paths, catalog.json rows, README counts 21
• CI Validate + blender-smoke 4.5 / 5.2 / labeled 5.1 + Socket green
• Apply needs-5.1 via REST and confirm the 5.1 job appears

Binaries: .scratch/blender-4.5.11-windows-x64 (4.5.11 LTS), .scratch/blender-5.1.2-windows-x64 (5.1.2), .scratch/blender-5.2.1-windows-x64 (5.2.1 LTS).

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