feat: add anvil and water-trough showcase pieces

[TMHSDigital]

Summary

Ordered showcase backlog is exhausted. Named picks: anvil, then water-trough. Next two (restocked on ROADMAP): hitching-post, grindstone.

One branch, one commit per piece. Showcase 13 → 15. 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: .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.

anvil (live run)

London-pattern anvil on a hooped timber stump. Closed-form, no RNG. Two 5.2.1 defaults matched.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	980–1140	1052 / 1052 / 1052
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2186 / 0.2186 / 0.2110
Materials	2, ≥24 metal	2 slots, 102 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(0.750, 0.406, 0.690) ± 0.01	(0.7500, 0.4060, 0.6900), zmin 0
Collider	≤ 180	134
Export	> 0 B	84092 / 84092 / 84088

	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 0.9s / 1.0s / 1.1s.

water-trough (live run)

Staved U-trough, both end boards, iron straps, trestle legs with shoes. Two 5.2.1 defaults matched.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	3880–4140	4008 / 4008 / 4008
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2196 / 0.2196 / 0.2196
Materials	2, ≥24 metal	2 slots, 168 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(1.136, 0.482, 0.470) ± 0.01	(1.1360, 0.4820, 0.4701), zmin 0
Collider	≤ 150	108
Export	> 0 B	292484 / 292484 / 292468

	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 2.3s / 1.6s / 1.8s.

DECIMATE COLLAPSE is the named cross-version tolerance (ratio band). Bake pixels stochastic (FINISHED + has_data). 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
• Catalog + plugin.json showcase + README counts 15
• 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