io: write element faces as binary and ASCII STL

Model::skin_faces gives the mesh's outer skin: the exterior faces of
the solids and the top side of every shell.

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

crates/io/src/lib.rs

@@ -27,6 +27,7 @@ pub mod memory;
 pub mod mesh;
 pub mod recovery;
 pub mod roots;
+pub mod stl;
 
 use std::collections::{BTreeMap, BTreeSet};
 use std::io::{Cursor, Read, Write};