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);

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,

src/app/update.rs

@@ -288,6 +288,48 @@ impl H7CAD {
 
             Message::StlExportPath(None) => Task::none(),
 
+            // ── STEP AP203 export ─────────────────────────────────────────
+            Message::StepExport => {
+                let i = self.active_tab;
+                if self.tabs[i].scene.meshes.is_empty() {
+                    self.command_line.push_error("STEPOUT: no 3D mesh data in this drawing.");
+                    return Task::none();
+                }
+                Task::perform(
+                    async {
+                        rfd::AsyncFileDialog::new()
+                            .set_title("Export STEP AP203")
+                            .set_file_name("export.step")
+                            .add_filter("STEP Files", &["step", "stp"])
+                            .add_filter("All Files", &["*"])
+                            .save_file()
+                            .await
+                            .map(|h| h.path().to_path_buf())
+                    },
+                    Message::StepExportPath,
+                )
+            }
+
+            Message::StepExportPath(Some(path)) => {
+                let i = self.active_tab;
+                let meshes: Vec<crate::scene::mesh_model::MeshModel> =
+                    self.tabs[i].scene.meshes.values().cloned().collect();
+                let mesh_refs: Vec<&crate::scene::mesh_model::MeshModel> = meshes.iter().collect();
+                match crate::io::step::build_step(&mesh_refs) {
+                    Some(text) => match std::fs::write(&path, text.as_bytes()) {
+                        Ok(()) => self.command_line.push_output(&format!(
+                            "STEPOUT: exported to \"{}\"",
+                            path.display()
+                        )),
+                        Err(e) => self.command_line.push_error(&format!("STEPOUT: write error: {e}")),
+                    },
+                    None => self.command_line.push_error("STEPOUT: no mesh data to export."),
+                }
+                Task::none()
+            }
+
+            Message::StepExportPath(None) => Task::none(),
+
             // ── OBJ import ────────────────────────────────────────────────
             Message::ObjImport => {
                 Task::perform(