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/progress.rs

@@ -288,7 +288,7 @@ mod tests {
         let mut s = Session::default();
         s.execute("file.sample", &json!({"name": "bracket-hex"})).unwrap();
         assert_eq!(status(&s, Stage::Mesh), Done);
-        let edge = json!({"body": "Bracket (1) (1)' (1)", "edge": "F17:face:5|F2:side:l1"});
+        let edge = json!({"body": "Bracket.17", "edge": "F17:face:5|F2:side:l1"});
         s.execute("mesh.seed.edge", &json!({"edges": [edge], "count": 6})).unwrap();
         assert_eq!(status(&s, Stage::Mesh), Outdated);
         let why = s.mesh_outdated().unwrap();