feat: add watchtower and cart showcase pieces

[TMHSDigital]

Summary

• watchtower: timber lookout (corner posts, X-braces, plank platform, three-sided rail, ladder, pyramidal roof, iron shoes) then UVs / high-to-low bake / LOD / convex collider / Unity glTF.
• cart: two-wheel wooden cart (slatted bed, side walls, shafts, closed-form torus rims, 8 spokes, iron hubs and axle) through the same pipeline.
• Numbered showcase list is exhausted. Next backlog after this pair: park bench, wheelbarrow.

Test plan

Local binaries (exact, self-reported): .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. Two consecutive 5.2.1 defaults matched. argparse --nope exits 2 on all three. --skip-decimate exits 9 on LOD1 on all three. Framing via check_framing (no deviation=). Heroes rendered Cycles on 5.2.1.

watchtower

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).

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	4550–4760	4656 / 4656 / 4656
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2199 / 0.2199 / 0.2199
Materials	2 distinct, ≥24 metal	60 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(0.910, 0.927, 1.822) ± 0.01	(0.9104, 0.9267, 1.8220), zmin 0
Collider	≤ 96	64
Export	> 0	334288 / 334288 / 334272

Default 0 / falsifier 9 / argparse 2 on all three. Falsifier violates LOD1 ratio. LOD2 ratio happened to match on this mesh; the gate is still a ratio band. Smoke wall-clock (check-only): 2873 / 1923 / 2017 ms.

cart

Composes skills mesh-editing-and-bmesh, bake-high-to-low, depsgraph-and-evaluated-data, engine-export-presets; same snippets (copied). Wheel rims are a closed-form torus (bmesh.ops.create_torus is not present).

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	2470–2680	2572 / 2572 / 2572
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2193 / 0.2193 / 0.2146
Materials	2 distinct, ≥24 metal	100 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(1.547, 0.749, 0.656) ± 0.01	(1.5473, 0.7486, 0.6560), zmin 0
Collider	≤ 360	306
Export	> 0	202500 / 202500 / 202492

Default 0 / falsifier 9 / argparse 2 on all three. Falsifier violates LOD1 ratio. 5.2.1 LOD2 is more aggressive (band, not exact count). Smoke wall-clock: 1391 / 1152 / 1383 ms.

Live run: default, --skip-decimate, argparse, two 5.2 defaults, Cycles stills, build_gallery.py alts (full sentence; data-tags includes showcase).
Inspection only: CI 4.5 series float vs local 4.5.11; Socket.

Smoke added

Per version ~4.26 / 3.08 / 3.40 s. Prior cumulative ~33 / 23 / 22 s → ~37 / 26 / 25 s. Under the ~90 s/version cap.