Export 3MF with B-rep face tags (#834)

3MF export carries each triangle's B-rep face tag.

crates/engine/src/cmd/file.rs

@@ -45,7 +45,7 @@ pub static COMMANDS: &[CommandSpec] = &[
     CommandSpec::new("file.export", "Export", export)
         .icon("export")
         .noundo()
-        .params("path; format?: stl|stla|obj|step|3mf|json (default from extension); bodies?: [names] (JSON exports the complete design)"),
+        .params("path; format?: stl|stla|obj|step|3mf|3mf-tagged|json (default from extension; 3mf-tagged: 3MF whose triangles carry their B-rep face, at full precision, for meshing and analysis tools; docs/formats/3mf-face-tags.md); bodies?: [names] (JSON exports the complete design)"),
     CommandSpec::new("file.save_mesh", "Save As Mesh", save_stl).icon("export").noundo().params("path; bodies?: [names]; ascii?: bool"),
 ];