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>

src/app/commands.rs

@@ -3185,6 +3185,11 @@ impl H7CAD {
                 }
             }
 
+            // ── STL export ────────────────────────────────────────────────
+            "STLOUT"|"EXPORTSTL" => {
+                return Task::done(Message::StlExport);
+            }
+
             // ── Plot / Page Setup ──────────────────────────────────────────
             "PRINT"|"PLOT"|"EXPORT" => {
                 return Task::done(Message::PlotExport);

src/app/mod.rs

@@ -435,6 +435,11 @@ pub enum Message {
     DataExtractionSave(String),
     /// Path chosen (or None = cancelled).
     DataExtractionSaveResult(String, Option<std::path::PathBuf>),
+    // ── STL export ────────────────────────────────────────────────────────
+    /// Trigger STL export: collect meshes and show save dialog.
+    StlExport,
+    /// Callback after the user picks (or cancels) the STL save path.
+    StlExportPath(Option<std::path::PathBuf>),
 }
 
 impl H7CAD {

src/app/update.rs

@@ -246,6 +246,48 @@ impl H7CAD {
 
             Message::DataExtractionSaveResult(_, None) => Task::none(),
 
+            Message::StlExport => {
+                let i = self.active_tab;
+                if self.tabs[i].scene.meshes.is_empty() {
+                    self.command_line.push_error("STLOUT: no 3D mesh data in this drawing.");
+                    return Task::none();
+                }
+                Task::perform(
+                    async {
+                        rfd::AsyncFileDialog::new()
+                            .set_title("Export STL")
+                            .set_file_name("export.stl")
+                            .add_filter("STL Files", &["stl"])
+                            .add_filter("All Files", &["*"])
+                            .save_file()
+                            .await
+                            .map(|h| h.path().to_path_buf())
+                    },
+                    Message::StlExportPath,
+                )
+            }
+
+            Message::StlExportPath(Some(path)) => {
+                // Re-build STL bytes (we can't easily pass them through the message).
+                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::stl::build_stl(&mesh_refs) {
+                    Some(bytes) => match std::fs::write(&path, bytes) {
+                        Ok(()) => self.command_line.push_output(&format!(
+                            "STLOUT: exported to \"{}\"",
+                            path.display()
+                        )),
+                        Err(e) => self.command_line.push_error(&format!("STLOUT: write error: {e}")),
+                    },
+                    None => self.command_line.push_error("STLOUT: no mesh data to export."),
+                }
+                Task::none()
+            }
+
+            Message::StlExportPath(None) => Task::none(),
+
             Message::SaveFile => {
                 let i = self.active_tab;
                 if let Some(path) = &self.tabs[i].current_path {