Straight edges stay one edge after cuts (#1049) (#1101)

* Straight edges stay one edge after cuts (#1049)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Robustness goldens: cuts join straight edges along untouched walls (#1049)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fmt

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Robustness goldens note: what each changed seed shows when replayed on both builds

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

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

apps/solidcraft-cli/tests/straight_edges_after_cuts.rs

@@ -0,0 +1,88 @@
+//! Straight edges stay one edge after many cuts (#1049). The owner's design
+//! (`tests/action_matrix/designs/crazy-geom-cuts.solidcraft`, saved by 0.10.0): five extrudes,
+//! joins and cuts, over four sketches, three of them on faces of the body with the faces'
+//! edges projected. Cuts used to leave the straight edges of the walls they sliced split at
+//! vertices nothing else used, so an edge that is one straight line was drawn (and lit on hover)
+//! as many pieces, and a sketch on such a face projected every piece.
+
+use std::collections::HashMap;
+use std::path::{Path, PathBuf};
+
+use serde_json::json;
+use solidcraft_engine::Session;
+use solidcraft_engine::doc::kernel::{FaceSurface, face_surface};
+use solidcraft_engine::geom::Vec3;
+
+fn design() -> PathBuf {
+    Path::new(env!("CARGO_MANIFEST_DIR")).join("../../tests/action_matrix/designs/crazy-geom-cuts.solidcraft")
+}
+
+fn open(path: &Path) -> Session {
+    let mut s = Session::default();
+    s.execute("doc.open", &json!({"path": path.to_string_lossy()})).unwrap_or_else(|e| panic!("{}: {e}", path.display()));
+    s
+}
+
+/// The design built from scratch: saved without cached geometry and opened again.
+fn rebuilt() -> Session {
+    let s = open(&design());
+    let dir = std::env::temp_dir().join(format!("solidcraft-straight-edges-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let path = dir.join("crazy-geom-cuts.json");
+    let mut t = s.scratch();
+    t.execute("file.save", &json!({"path": path.to_string_lossy(), "format": "json"})).unwrap();
+    let out = open(&path);
+    let _ = std::fs::remove_dir_all(&dir);
+    out
+}
+
+fn errors(s: &Session) -> Vec<(u64, String)> {
+    s.model.results.iter().filter_map(|r| r.error.clone().map(|e| (r.id, e))).collect()
+}
+
+fn key(v: Vec3) -> [i64; 3] {
+    [v.x, v.y, v.z].map(|c| (c * 1e5).round() as i64)
+}
+
+#[test]
+fn straight_edges_are_one_edge_after_many_cuts() {
+    for (what, s) in [("as saved", open(&design())), ("built from scratch", rebuilt())] {
+        assert_eq!(errors(&s), vec![], "{what}: every feature evaluates");
+        let st = s.world_state();
+        let bodies: Vec<_> = st.bodies.iter().filter(|b| !b.body.is_mesh()).collect();
+        assert_eq!(bodies.len(), 1, "{what}: one body");
+        let body = &bodies[0].body;
+        let mesh = body.tessellate(0.01).unwrap();
+        // Every edge is drawn as one polyline; a straight one as one segment.
+        let mut at: HashMap<[i64; 3], Vec<usize>> = HashMap::new();
+        for (i, e) in mesh.edges.iter().enumerate() {
+            let (Some(a), Some(z)) = (e.first(), e.last()) else { panic!("{what}: edge {i} is empty") };
+            at.entry(key(*a)).or_default().push(i);
+            at.entry(key(*z)).or_default().push(i);
+        }
+        let straight = |i: usize| mesh.edges.get(i).filter(|e| e.len() == 2).and_then(|e| (e[1] - e[0]).normalized());
+        let split: Vec<_> = at
+            .iter()
+            .filter(|(_, es)| match es.as_slice() {
+                [a, b] => straight(*a).zip(straight(*b)).is_some_and(|(u, w)| u.dot(w).abs() > 1. - 1e-9),
+                _ => false,
+            })
+            .map(|(k, _)| k.map(|c| c as f64 / 1e5))
+            .collect();
+        assert_eq!(split, Vec::<[f64; 3]>::new(), "{what}: straight edges split where nothing else meets them");
+        // No two faces side by side in one plane.
+        let plane = |f: usize| match face_surface(body, f) {
+            Some(FaceSurface::Plane { normal, point }) => Some((normal, normal.dot(point))),
+            _ => None,
+        };
+        for (i, faces) in mesh.edge_faces.iter().enumerate() {
+            if let [a, b] = faces.as_slice()
+                && a != b
+                && let (Some((na, da)), Some((nb, db))) = (plane(*a as usize), plane(*b as usize))
+            {
+                assert!(na.dot(nb) < 1. - 1e-9 || (da - db).abs() > 1e-6, "{what}: faces {a} and {b} meet at edge {i} in one plane");
+            }
+        }
+        assert_eq!((body.face_count(), body.edge_count(), body.vertex_count()), (35, 107, 71), "{what}: faces, edges and vertices");
+    }
+}