io: write VTK XML unstructured grids and PVD collections

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

crates/io/src/lib.rs

@@ -28,6 +28,7 @@ pub mod mesh;
 pub mod recovery;
 pub mod roots;
 pub mod stl;
+pub mod vtk;
 
 use std::collections::{BTreeMap, BTreeSet};
 use std::io::{Cursor, Read, Write};