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>

ROADMAP.md

@@ -295,7 +295,8 @@ Underlay (PDF/DWF/DGN)
 | Boolean operasyonlar (UNION/SUBTRACT/INTERSECT) | ⬜ |
 | EXTRUDE / REVOLVE / SWEEP / LOFT | ⬜ |
 | 3D ARRAY | ✅ |
-| STL / STEP dışa aktarma | ⬜ |
+| STL dışa aktarma (STLOUT) | ✅ |
+| STEP dışa aktarma | ⬜ |
 
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