feat: STL export (STLOUT / EXPORTSTL command)

Exports all tessellated 3D mesh models in the current drawing to a
binary STL file via a save-file dialog. Normals are taken from the
MeshModel where available, otherwise computed from triangle vertices.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/io/mod.rs

@@ -5,6 +5,7 @@
 
 pub mod pdf_export;
 pub mod plot_style;
+pub mod stl;
 pub mod xref;
 
 use acadrust::io::dwg::DwgReader;