Add FrozenGraph tests

src/models/frozen_graph.rs

@@ -248,3 +248,70 @@ impl GraphTopology for FrozenGraph {
         }
     }
 }
+
+#[cfg(test)]
+mod trait_tests {
+    use super::FrozenGraph;
+
+    crate::graph_topology_tests!(FrozenGraph);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::testing::fixtures::MakeTestGraph;
+    use crate::traits::GraphTopology;
+
+    #[test]
+    fn incident_vertices_opposite_handle() {
+        let (graph, [v0, v1], e) = FrozenGraph::single_edge();
+        let f = Edge(graph.incidences[e.0].opposite);
+        let (u0, u1) = graph.incident_vertices(f);
+        assert!(
+            (u0 == v0 && u1 == v1) || (u0 == v1 && u1 == v0),
+            "unexpected incident vertices {u0:?} and {u1:?} for edge {f:?}"
+        );
+    }
+
+    #[test]
+    fn csr_sentinel_and_offsets_standard() {
+        let (graph, ..) = FrozenGraph::standard();
+        assert_eq!(
+            *graph.vertices.last().unwrap(),
+            graph.incidences.len(),
+            "sentinel should equal total number of incidences"
+        );
+        assert!(
+            graph.vertices.windows(2).all(|w| w[0] <= w[1]),
+            "incidence offsets of vertices should be monotonically non-decreasing"
+        );
+    }
+
+    #[test]
+    fn csr_sentinel_and_offsets_empty() {
+        let graph = FrozenGraph::empty();
+        assert_eq!(
+            graph.vertices,
+            vec![0],
+            "empty graph should have a single sentinel entry of 0"
+        );
+        assert!(
+            graph.incidences.is_empty(),
+            "empty graph should have no incidences"
+        );
+    }
+
+    #[test]
+    fn opposite_symmetry() {
+        let (graph, ..) = FrozenGraph::standard();
+        for i in 0..graph.incidences.len() {
+            let opp = graph.incidences[i].opposite;
+            assert_ne!(opp, i, "incidence should not equal its own opposite");
+            assert_eq!(
+                graph.incidences[opp].opposite, i,
+                "opposite of opposite of incidence should equal the incidence"
+            );
+        }
+    }
+}