airliner mesh is generated in-repo: tools/plane_gen.py writes vendor/plane3d.js (480 tris, CC0, same unit extent as the old Grab3D derivative); attribution updated

vendor/LICENSE

@@ -8,10 +8,9 @@ stars.json - star catalogue derived from the Yale Bright Star Catalogue
 (5th ed., public catalogue data). Generated for this project; see the
 real-sky notes in clouds3d.js.
 
-plane3d.js - low-poly airliner mesh (599 triangles, flattened, no textures)
-Derived from 'Passenger Airplane' by Grab3D (https://grab3d.com), CC0 1.0
-(public domain dedication). Decimated and repacked for this project; the
-derived mesh is offered under CC0 as well.
+plane3d.js - low-poly airliner mesh (480 triangles, flat shaded, no textures)
+Original geometry generated by tools/plane_gen.py in this repository; offered
+under CC0 1.0 like the rest of the project's own assets.
 
 clouds3d.js - original code for this project (same license as the
 repository root LICENSE).