Add deterministic Blender yard-asset factory

One script regenerates every nature/hardware asset in PLAN3D §5-E, following
the house idiom from 3D-STORE/racks_to_glb.py: reset per asset, build under a
root empty at the origin, join by group, stamp props, export Y-up GLB.

Groups are joined per sway-unit rather than per-asset, so trees keep trunk and
canopy_* as separate nodes for Lane A to animate. Paths resolve from __file__
instead of a hardcoded home dir, since the library lives elsewhere on this box.

Verification re-imports each exported GLB from disk and asserts dims, tri
budget, and node-name survival, then renders it against the 1.7 m ref capsule.
Checking the file rather than the in-memory scene is what makes it a real test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,11 @@
+# Per-asset verification renders — regenerable, and 3 MB of churn.
+# The tiled contact_sheet.png IS committed; it's the acceptance evidence.
+tools/blender/thumbs/
+
+# PLAN3D §0: raw sources stay in the shared libraries, runtime asset is .glb
+*.blend
+*.blend1
+*.fbx
+
+.DS_Store
+__pycache__/