fix: rebuild four weak showcase props and collapse the README grid

[TMHSDigital]

Summary

• Rebuild iron-cauldron (shared-vertex lathe, not overlapping stave panels), wall-torch (dressed plaque + flame cluster), anvil (London-pattern bevel + 3/4), and shipping-crate (materials.clear() was wiping iron; corner brackets + end handles; 3/4 camera).
• Collapse README Showcase into a <details> with a 4-across width=24% grid so 24 full-bleed 1200x675 thumbs are not a scroll trap. Count strings stay at 24 showcase pieces.
• Budgets refit from the new meshes. --skip-decimate still exits 9. No examples/skills/snippets/rules/templates touched.

Live run (not inspection)

Binaries:

• E:\Blender-Developer-Tools\.scratch\blender-4.5.11-windows-x64\blender.exe → 4.5.11
• E:\Blender-Developer-Tools\.scratch\blender-5.1.2-windows-x64\blender.exe → 5.1.2
• E:\Blender-Developer-Tools\.scratch\blender-5.2.1-windows-x64\blender.exe → 5.2.1 (Blender 5.2.1 LTS (hash 9e2066aef7ef))

Default / --skip-decimate / --nope on all three series, all four pieces: 0 / 9 / 2.

Piece	base tris (4.5 / 5.1 / 5.2)	bbox m	collider	5.2 framing
iron-cauldron	3388 / 3388 / 3388	0.606×0.591×0.826	180	fill y=0.789, margins ok
wall-torch	848 / 848 / 848	0.384×0.360×0.670	92	fill y=0.878, margins ok
anvil	6616 / 6616 / 6616	0.772×0.419×0.662	190	fill y=0.733, margins ok
shipping-crate	624 / 624 / 624	1.352×0.856×0.748	36	fill y=0.790, margins ok (projection)

Hero/preview encoded Pillow webp q=85 method=6. python scripts/build_gallery.py — index alts and witnesses callouts match showcase/gallery.json (no dotted-path truncation).

Crate metal-face regression: materials.clear() reset every material_index to 0. Falsifier for that is now METAL_FACES_MIN=24 (exit 5). LOD falsifier remains --skip-decimate exit 9.

Test plan

• blender-smoke 5.2 LTS and 4.5 LTS green on this head
• blender-smoke 5.1 via needs-5.1
• README Showcase is collapsed by default; expand shows a compact 4-across grid
• Gallery cards for the four pieces show the new heroes (iron pot, sconce, horned anvil, crate with iron)

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