Sheet metal: sew the folded-material guard per piece so adjacent flanges cut (#296) (#355)

The guard that refuses machining through bends sewed every bend zone and
nonparallel panel into one solid. Two bends meeting at an open corner relief
share only an edge, so that sew failed with "nonmanifold edge contact" and any
cut on the base was refused. Sew and test each piece on its own instead.

Reference case 75-sm-bracket-holes now passes; its known_failure mark is removed.

Fixes #296


Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/sheet_tests.rs

@@ -1685,3 +1685,33 @@ fn fold_at_a_tab_root_names_the_real_cause() {
     outline_blank(&mut ok, json!([[0, 0], [100, 0], [100, 50], [60, 50], [60, 70], [40, 70], [40, 50], [0, 50]]));
     run(&mut ok, "sheet.fold", json!({"points": [[40, 51, 0.8], [60, 51, 0.8]], "fixed": [50, 25, 0.8]}));
 }
+
+/// #296: two bends meeting at an open corner relief touch only along an edge, so the folded
+/// material the machining guard checks must not be sewn into one solid.
+#[test]
+fn hole_beside_two_adjacent_edge_flanges_cuts_and_bends_stay_guarded() {
+    let build = |s: &mut Session| {
+        base(s, 80.0, 40.0);
+        run(s, "sheet.flange", json!({"edges":[[40.0, 0.0, 2.5]],"height":30,"angle":90}));
+        run(s, "sheet.flange", json!({"edges":[[80.0, 22.5, 2.5]],"height":15,"angle":90}));
+    };
+    let hole = |s: &mut Session, name: &str, centre: [f64; 2]| {
+        run(s, "sketch.create", json!({"plane":"XY","name":name}));
+        run(s, "sketch.circle.center", json!({"center":centre,"radius":3}));
+        run(s, "sketch.finish", json!({}));
+        s.execute("solid.extrude", &json!({"sketch":name,"through_all":true,"operation":"cut"}))
+    };
+    let mut s = Session::default();
+    build(&mut s);
+    let before = volume(&mut s);
+    assert!((before - 14057.2).abs() < 0.1, "{before}");
+    if let Err(e) = hole(&mut s, "Hole", [25.0, 20.0]) {
+        panic!("hole beside two flanges: {e}");
+    }
+    let after = volume(&mut s);
+    assert!((before - after - 9.0 * PI * 2.5).abs() < 0.05, "{before} -> {after}");
+    let mut bend = Session::default();
+    build(&mut bend);
+    let error = hole(&mut bend, "Bend", [78.5, 20.0]).unwrap_err().to_string();
+    assert!(error.contains("bend"), "{error}");
+}