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/app/commands.rs

@@ -3788,6 +3788,11 @@ impl H7CAD {
                 return Task::done(Message::StlExport);
             }
 
+            // STEPOUT — export 3D meshes to STEP AP203 format
+            "STEPOUT"|"EXPORTSTEP"|"STPOUT" => {
+                return Task::done(Message::StepExport);
+            }
+
             // ── Plot Style Editor GUI ─────────────────────────────────────
             "PLOTSTYLEPANEL"|"PLOTSTYLEEDITOR"|"STYLESMANAGER" => {
                 return Task::done(Message::PlotStylePanelOpen);