geometry: number the pieces of a split body by position (#349)

SolidCraft names the pieces of a split body by stacking suffixes
(Bracket (1) (1)' (1)). On opening a design, the pieces of one body are
named <body>.1 … <body>.N, ordered by centroid, so the tree, tooltips,
forms and picks read as one part with numbered cells.

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

crates/engine/src/cmd/hex.rs

@@ -523,7 +523,7 @@ mod tests {
         let (_dir, mut s) = split_box(true);
         let before = s.model.clone();
         let e = s.execute("mesh.hex.blocks", &json!({"size": 2.5})).unwrap_err().to_string();
-        assert!(e.contains("do not conform") && e.contains("`Box`"), "{e}");
+        assert!(e.contains("do not conform") && e.contains("`Box.1`"), "{e}");
         assert_eq!(s.model, before);
         for p in [
             json!({"size": 0}),