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/frames.rs

@@ -67,6 +67,7 @@ fn table(id: &str) -> &'static [(&'static str, K)] {
         "sheet.convert" => &[("face", Pick), ("bends", Picks), ("body", Body)],
         "sheet.flange" | "sheet.hem" => &[("edges", Picks), ("body", Body)],
         "sheet.fold" => &[("points", Picks), ("fixed", Pick), ("body", Body)],
+        "sheet.perforate" => &[("body", Body)],
         "sketch.create" | "sketch.redefine" => &[("plane", HerePlane)],
         "canvas.decal" => &[("face", Here)],
         "canvas.insert" => &[("plane", HerePlane), ("at", Here)],