feat: add terrain-scatter and fence-kit showcase pieces

[TMHSDigital]

Summary

• terrain-scatter: Geometry Nodes sine-hill Mesh Grid + Index-jittered Instance-on-Points scatter, realized cubes replaced with bevelled masonry, then UVs / high-to-low bake / LOD / convex collider / Unity glTF.
• fence-kit: tiling post-and-rail section (two posts, pyramidal caps, three rails, kickboard, brace, iron shoes and straps). AABB X is the tile width; does not re-witness modular-kit-snap.
• Next backlog after this pair: watchtower, cart.

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.

terrain-scatter

Composes skills geometry-nodes-python, 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	1150–1250	1194 / 1194 / 1194
LOD1	0.32–0.62	0.4992 / 0.4992 / 0.4992
LOD2	0.10–0.35	0.2194 / 0.2194 / 0.2194
Materials	2 distinct, ≥24 stone	486 stone
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(1.800, 1.800, 0.655) ± 0.01	(1.8000, 1.8000, 0.6554), zmin 0
Collider	≤ 80	52
Export	> 0	90704 / 90704 / 90696

Default 0 / falsifier 9 / argparse 2 on all three. Falsifier violates LOD1 ratio. Smoke wall-clock (check-only): 994 / 970 / 1080 ms.

fence-kit

Composes skills mesh-editing-and-bmesh, bake-high-to-low, depsgraph-and-evaluated-data, engine-export-presets; same snippets (copied).

Axis	Declared	4.5.11 / 5.1.2 / 5.2.1
Base tris	1160–1280	1212 / 1212 / 1212
LOD1	0.32–0.62	0.5000 / 0.5000 / 0.5000
LOD2	0.10–0.35	0.2195 / 0.2195 / 0.2079
Materials	2 distinct, ≥24 metal	120 metal
UVs	0..1, overlap ≤ 1e-5	in range, 0
AABB	(1.642, 0.157, 1.235) ± 0.01	(1.6418, 0.1574, 1.2350), zmin 0
Collider	≤ 96	62
Export	> 0	93232 / 93232 / 93224

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: 961 / 888 / 1053 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 4.5.13 float vs local 4.5.11; Socket.

Smoke added

Per version ~1.96 / 1.86 / 2.13 s. Prior cumulative ~31 / 21 / 20 s → ~33 / 23 / 22 s. Under the ~90 s/version cap.

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