feat(io): FBXEXPORT writes the drawing's 3D meshes as binary FBX

FBXEXPORT (also EXPORTFBX) saves every 3D mesh of the drawing (solids,
surfaces and meshes, at full display resolution) as a binary FBX 7.4
file for visualisation tools: one model per object with its geometry,
face normals and a material in its display colour. Axes stay Z-up and
the unit scale follows the drawing's insertion units.

Refs #1502

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/io/mod.rs

@@ -25,6 +25,7 @@ pub mod print_to_printer;
 pub mod recovery;
 #[cfg(not(target_arch = "wasm32"))]
 pub mod single_instance;
+pub mod fbx;
 pub mod step;
 pub mod stl;
 pub mod svg_export;