geometry: stitch, a surface's gaps under a tolerance closed (#507)

Open points closer than the tolerance become one point and a point on
an open edge near the inside of another splits that edge's triangle;
edges that are not open are never touched.

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

crates/geometry/src/lib.rs

@@ -25,6 +25,7 @@ pub mod partition;
 pub mod sample;
 #[cfg(feature = "scripts")]
 mod script;
+pub mod stitch;
 pub mod virtual_faces;
 
 pub use design::{Design, FeatureError, MAX_DESIGN_BYTES, open_design};