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/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 {