Sheet metal: sheet.perforate, a perforation field exported to the flat pattern and DXF (#732) (#1041)

Adds sheet.perforate: round holes on a staggered or square grid over the flat sheet, kept clear of edges, cut-outs and bend zones. The holes go to the flat pattern, the DXF and the measured volume rather than the B-rep, so a panel can carry many thousands of them.

crates/engine/src/cmd/sketch_modify_tests.rs

@@ -1475,7 +1475,7 @@ fn offset_driver_format18_offsets_keep_fixed_distances_on_upgrade() {
     let mut json: Value = serde_json::from_str(&doc.to_json()).unwrap();
     json["format"] = json!("solidcraft/18");
     let mut reopened = Session::new(solidcraft_doc::Document::from_json(&json.to_string()).unwrap());
-    assert_eq!(reopened.doc.format, "solidcraft/20");
+    assert_eq!(reopened.doc.format, solidcraft_doc::format::format_string(solidcraft_doc::format::FORMAT), "upgraded on opening");
     run(&mut reopened, "sketch.edit", json!({"sketch":1}));
     let sk = sketch(&reopened);
     assert!((sk.offsets[0].distance - 2.0).abs() < 1e-8);