engine: mesh.export.stl writes the outer surface or a set's faces

Binary by default, ASCII on request; in the Mesh tab's Verify panel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/mod.rs

@@ -74,6 +74,7 @@ pub static ALL: &[CommandSpec] = &[
     mesh::ELEMENT_REMOVE,
     replace::CLEAR,
     mesh::QUALITY,
+    mesh::EXPORT_STL,
     mesh::DELAUNAY_POINTS,
     mesh::SURFACE_CHECK,
     mesh::PLANAR,