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/mod.rs

@@ -497,6 +497,11 @@ pub enum Message {
     StlExport,
     /// Callback after the user picks (or cancels) the STL save path.
     StlExportPath(Option<std::path::PathBuf>),
+    // ── STEP export ───────────────────────────────────────────────────────
+    /// Trigger STEP AP203 export: show save dialog.
+    StepExport,
+    /// Callback after the user picks (or cancels) the STEP save path.
+    StepExportPath(Option<std::path::PathBuf>),
     // ── OBJ import ────────────────────────────────────────────────────────
     /// Trigger OBJ import: show open-file dialog.
     ObjImport,