Mesh: mirror elements across a plane, right-handed, sharing the plane's nodes

fieldcraft_mesh::mirror reflects a region's tet4, tet10, wedge6, hex8 and hex20
elements across a plane. The copies list their nodes in reflected order (corners
swapped, mid-side nodes following their edges), so every copy is right-handed;
reflected_faces and reflected_edges give the copy's face and edge over the same
nodes. Copies of the nodes on the plane are merged with the originals through
join::merge_coincident, so the region and its image form one conforming mesh.

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

crates/mesh/src/lib.rs

@@ -12,6 +12,7 @@ pub mod cdt;
 pub mod delaunay;
 pub mod hex;
 pub mod join;
+pub mod mirror;
 pub mod octree;
 pub mod planar;
 pub mod quad;