feat: STEPOUT command — export 3D meshes to STEP AP203

Adds STEPOUT / STPOUT / EXPORTSTEP command that exports all tessellated
Solid3D / Region / Body meshes to an ISO 10303-21 (STEP AP203) file.

Each triangle is encoded as a minimal ADVANCED_FACE with a PLANE surface
and CLOSED_SHELL topology — sufficient for import into all major CAD systems.
A file-save dialog picks the output path; the command line reports success or
errors. Requires at least one tessellated solid in the drawing.

Closes ROADMAP 14.10.

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

src/io/mod.rs

@@ -7,6 +7,7 @@ pub mod obj;
 pub mod pdf_export;
 pub mod plot_style;
 pub mod print_to_printer;
+pub mod step;
 pub mod stl;
 pub mod xref;