STL export welds shared vertices like the 3MF (#816)

STL (binary and ASCII) now welds each body's export mesh with the same tolerance as 3MF, so filleted and drilled bodies are closed. The weld grid's neighbour lookup no longer overflows for bodies far from the origin.

Fixes #784

crates/io/src/threemf.rs

@@ -109,7 +109,7 @@ pub fn weld_tagged(positions: &[Vec3], triangles: &[[u32; 3]], tags: &[u32], eps
         'search: for dx in -1..=1 {
             for dy in -1..=1 {
                 for dz in -1..=1 {
-                    for &i in grid.get(&[c[0] + dx, c[1] + dy, c[2] + dz]).into_iter().flatten() {
+                    for &i in grid.get(&[c[0].saturating_add(dx), c[1].saturating_add(dy), c[2].saturating_add(dz)]).into_iter().flatten() {
                         if verts.get(i as usize).is_some_and(|q| q.dist(*p) <= eps) {
                             found = Some(i);
                             break 'search;
@@ -798,4 +798,15 @@ mod tests {
         assert!(objs.len() < 4_000, "the shared budget must stop the later items: {} built", objs.len());
         assert!(warnings.iter().any(|w| w.contains("too many triangles")), "{warnings:?}");
     }
+
+    /// A body far from the origin (a pattern spacing of "1e12 mm" gets there) welds without
+    /// overflowing the grid's cell indices.
+    #[test]
+    fn weld_survives_extreme_coordinates() {
+        let o = Vec3::new(1e12, 0.0, 0.0);
+        let positions = [o, o + Vec3::new(0.1, 0.0, 0.0), o + Vec3::new(0.0, 0.1, 0.0), o + Vec3::new(0.0, 0.0, 0.1)];
+        let triangles = [[0, 2, 1], [0, 1, 3], [1, 2, 3], [2, 0, 3]];
+        let (v, t) = weld(&positions, &triangles, 1e-7);
+        assert_eq!((v.len(), t.len()), (4, 4));
+    }
 }