feat: OBJ mesh import (IMPORTOBJ / OBJIMPORT command)

Parses Wavefront OBJ files (v/vn/f, fan-triangulation, face normals
fallback) and inserts the result as a Solid3D placeholder + MeshModel
visible in the 3D viewport.

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

src/io/mod.rs

@@ -3,6 +3,7 @@
 // All file reading/writing goes through acadrust.
 // Default save format: DWG (AC1032 / R2018+).
 
+pub mod obj;
 pub mod pdf_export;
 pub mod plot_style;
 pub mod stl;