Convert to Sheet Metal keeps a plate's round holes round (#977)

The plate check compared the face outline's area (from the tessellation, so each hole was a polygon of chords) with the exact volume, and refused plates with a few small holes. Where the outline runs along a cylinder about the plate's normal it is now an arc of that circle; coaxial pieces of one radius are one wall, so a hole split into two, three or four faces is one circle. The check passes with its tolerance unchanged, and the flat pattern and its DXF carry one CIRCLE per hole (ARC for rounded corners and slot ends).

A round blank now converts too, so unfold_cut_refold_and_convert checks the refusal with a block thicker at one end instead of a rod.

crates/engine/src/cmd/sheet_tests.rs

@@ -494,9 +494,17 @@ fn unfold_cut_refold_and_convert() {
     run(&mut s, "sheet.convert", json!({"body": "Plate", "face": [50, 30, 2.5]}));
     run(&mut s, "sheet.flange", json!({"edges": [[50, 0, 2.5]], "height": 20}));
     assert!(rel(volume(&mut s), 18972.622) < 1e-4, "{}", volume(&mut s));
+    // A rod picked on its end is a round plate as thick as the rod is long (as a 20 x 20 x 30 bar
+    // is a square one); a block thicker at one end is no plate.
     let mut t = Session::default();
     run(&mut t, "solid.cylinder", json!({"radius": 10, "height": 30, "body_name": "Rod"}));
-    assert!(t.execute("sheet.convert", &json!({"body": "Rod", "face": [0, 0, 30]})).is_err());
+    run(&mut t, "sheet.convert", json!({"body": "Rod", "face": [0, 0, 30]}));
+    let f = flat(&mut t);
+    assert!(rel(f["thickness"].as_f64().unwrap_or(0.0), 30.0) < 1e-9 && rel(f["flat_size_mm"][0].as_f64().unwrap_or(0.0), 20.0) < 1e-4, "{f}");
+    let mut t = Session::default();
+    run(&mut t, "solid.box", json!({"length": 100, "width": 60, "height": 2.5, "body_name": "Block"}));
+    run(&mut t, "solid.box", json!({"corner": [0, 0, 0], "length": 20, "width": 60, "height": 10, "operation": "join"}));
+    assert!(t.execute("sheet.convert", &json!({"body": "Block", "face": [50, 30, 2.5]})).is_err());
 }
 
 /// A sketch line across a 100 x 60 plate's top face at x = 40.