test: failing regression test for #264: Sheet metal: collinear tabs cannot be folded or flanged together (#279)

* test: reproduce #264 (Sheet metal: collinear tabs cannot be folded or flanged together)

* fix(sheet): fold and flange tabs that sit side by side on one line (#264)

A fold's bend zone and a flange's setback were cut with an infinite line, so
a second tab on the same line blocked a fold ('slanted or curved edge') and
two tab-end flanges in one feature shared one trimmed edge (nonmanifold).
When the zone falls apart into pieces, fold only the pieces the fold points
span, each as its own bend; trim a flange's setback only from the material
along its own edge, and attach each flange to the edge it was picked on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

---------

Co-authored-by: bherbruck <brennen.herbruck@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/sheet_tests.rs

@@ -1568,6 +1568,49 @@ fn outline_blank(s: &mut Session, outline: Value) {
     run(s, "sheet.flange", json!({"sketch": "Blank"}));
 }
 
+/// A 100 x 40 blank with two 15 mm tabs on its right edge (y 5..12 and y 28..35), on one bend line.
+fn two_tab_blank(s: &mut Session) {
+    outline_blank(
+        s,
+        json!([[0, 0], [100, 0], [100, 5], [115, 5], [115, 12], [100, 12], [100, 28], [115, 28], [115, 35], [100, 35], [100, 40], [0, 40]]),
+    );
+}
+
+/// Folding one tab must not be refused because another tab lies on the same line (#264).
+#[test]
+fn fold_one_of_two_collinear_tabs() {
+    let mut s = Session::default();
+    two_tab_blank(&mut s);
+    let flat = volume(&mut s);
+    run(&mut s, "sheet.fold", json!({"points": [[107, 5, 0.8], [107, 12, 0.8]], "fixed": [50, 20, 0.8]}));
+    assert!(rel(volume(&mut s), flat) < 1e-3, "a fold keeps the material");
+}
+
+/// One fold line across both tabs bends both (#264).
+#[test]
+fn fold_across_two_collinear_tabs() {
+    let mut s = Session::default();
+    two_tab_blank(&mut s);
+    let flat = volume(&mut s);
+    run(&mut s, "sheet.fold", json!({"points": [[107, 5, 0.8], [107, 35, 0.8]], "fixed": [50, 20, 0.8]}));
+    assert!(rel(volume(&mut s), flat) < 1e-3);
+    let bb = &run(&mut s, "inspect.measure", json!({}))["bodies"][0]["bbox"];
+    assert!(bb["max"][2].as_f64().unwrap() > 7.0, "both tabs stand up: {bb}");
+}
+
+/// Edge flanges on both tab ends in one feature (two separate features already work) (#264).
+#[test]
+fn edge_flanges_on_two_collinear_tab_ends_in_one_feature() {
+    let mut s = Session::default();
+    two_tab_blank(&mut s);
+    run(&mut s, "sheet.flange", json!({"edges": [[115, 8, 0.8], [115, 31, 0.8]], "height": "10"}));
+    let mut separate = Session::default();
+    two_tab_blank(&mut separate);
+    run(&mut separate, "sheet.flange", json!({"edges": [[115, 8, 0.8]], "height": "10"}));
+    run(&mut separate, "sheet.flange", json!({"edges": [[115, 31, 0.8]], "height": "10"}));
+    assert!(rel(volume(&mut s), volume(&mut separate)) < 1e-6);
+}
+
 /// `fixed` inside a cut-out touches no material: the fold must refuse it or say which side it kept (#263).
 #[test]
 fn fold_fixed_point_off_the_sheet_is_not_silently_ignored() {