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/doc/src/feature_refs.rs

@@ -163,6 +163,7 @@ macro_rules! feature_walk {
                 | FeatureKind::DeleteFace { body, .. }
                 | FeatureKind::SheetFlange { body, .. }
                 | FeatureKind::SheetHem { body, .. }
+                | FeatureKind::SheetPerforate { body, .. }
                 | FeatureKind::SheetUnfold { body, .. }
                 | FeatureKind::Boss { body, .. }
                 | FeatureKind::Lip { body, .. }