feat: add hitching-post and grindstone showcase pieces

[TMHSDigital]

Summary

Ordered showcase backlog is exhausted. Named picks: hitching-post, then grindstone. Next two (restocked on ROADMAP): hand-pump, signpost.

One branch, one commit per piece. Showcase 15 → 17. 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.

hitching-post (live run)

Square timber post, pyramidal cap, cross-arm, iron shoe, two wrapping bands, two hitching rings. Closed-form, no RNG. Two 5.2.1 defaults matched.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	780–920	848 / 848 / 848
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2193 / 0.2193 / 0.1958
Materials	2, ≥24 metal	2 slots, 262 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(0.460, 0.194, 1.260) ± 0.01	(0.4600, 0.1943, 1.2600), zmin 0
Collider	≤ 180	144
Export	> 0 B	73164 / 73164 / 73160

	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 1.1s / 0.9s / 1.1s.

grindstone (live run)

Stone wheel on a timber A-frame, iron axle, hubs, crank, tray. Two 5.2.1 defaults matched.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	1480–1720	1604 / 1604 / 1604
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2195 / 0.2195 / 0.2195
Materials	3, ≥24 metal and stone	3 slots, 80 metal, 26 stone
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(0.630, 0.615, 0.575) ± 0.01	(0.6300, 0.6150, 0.5750), zmin 0
Collider	≤ 180	130
Export	> 0 B	123436 / 123436 / 123416

	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 1.0s / 1.0s / 1.2s.

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 17
• 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