feat: add street-lantern and treasure-chest showcase pieces

[TMHSDigital]

Summary

Named showcase list (stone-well, wooden-barrel, campfire, market-stall) is exhausted. This PR picks the ROADMAP silhouette pair:

• street-lantern — hanging iron post lantern, brass collars, muntined amber cage, pyramidal roof
• treasure-chest — hollow slatted chest, iron bands, corner brackets, lid cracked open

Showcase count 5 → 7. Next backlog: GN terrain scatter, modular kit, then the newly restocked park bench / wheelbarrow.

Street lantern

Composes 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).

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	3400–3650	3520 / 3520 / 3520
LOD1 / LOD2	0.32–0.62 / 0.10–0.35	0.5000 / 0.2199 on all three
Materials	3 distinct, ≥8 glass, ≥8 brass	24 glass, 94 brass
AABB	(0.403, 0.892, 1.404) ± 0.01	(0.4032, 0.8916, 1.4041), zmin 0
Collider	≤ 110	84
Export	size > 0	254552 / 254552 / 254536 B

Exits (all three binaries): default 0, --skip-decimate 9 (LOD1 ratio 1.0), --nope 2. Falsifier violates the LOD1 ratio budget. Two consecutive 5.2.1 defaults matched. Added smoke wall-clock: 1.93s / 1.36s / 1.61s (4.5.11 / 5.1.2 / 5.2.1). Still rendered on 5.2.1; check_framing fill y=0.817, no deviation=.

Treasure chest

Same composed skills/snippets.

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	5600–5850	5724 / 5724 / 5724
LOD1 / LOD2	0.32–0.62 / 0.10–0.35	0.5000 / 0.2198 on all three
Materials	2 distinct, ≥24 metal	180 metal
AABB	(0.776, 0.549, 0.631) ± 0.01	(0.7760, 0.5486, 0.6312), zmin 0
Collider	≤ 160	138
Export	size > 0	416656 / 416656 / 416644 B

Exits (all three binaries): default 0, --skip-decimate 9 (LOD1 ratio 1.0), --nope 2. Falsifier violates the LOD1 ratio budget. Two consecutive 5.2.1 defaults matched. Added smoke wall-clock: 3.88s / 2.20s / 2.41s. Still rendered on 5.2.1; check_framing fill y=0.722, no deviation=.

DECIMATE COLLAPSE happened to match across series on both meshes; the gate is still a ratio band. glTF bytes differ by tens of bytes. Bake pixels are stochastic (FINISHED + has_data).

Evidence

Live run: budgets, default/falsifier/argparse exits, determinism pair, and 5.2.1 stills on .scratch/blender-{4.5.11,5.1.2,5.2.1}-windows-x64/blender.exe (versions the binaries reported). Inspection only: gallery HTML alts/tags after scripts/build_gallery.py.

Test plan

• Default 0 and --skip-decimate 9 on 4.5.11, 5.1.2, 5.2.1
• Argparse --nope exits 2 on all three
• Two 5.2.1 defaults identical
• needs-5.1 via REST; wait 4.5 + 5.1 + 5.2 smoke plus Socket
• CI green, then squash-merge

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