Add VertexMap tests for Graph (vertex deletion)

src/maps.rs

@@ -51,7 +51,8 @@ impl<V: Copy, T: Clone> IndexMut<V> for VertexMap<V, T> {
 #[cfg(test)]
 mod tests {
     use crate::models::append_graph::AppendGraph;
-    use crate::traits::GraphTopology;
+    use crate::models::graph::Graph;
+    use crate::traits::{GraphTopology, GraphTopologyDeletion};
 
     #[test]
     fn initial_values_are_default() {
@@ -119,4 +120,44 @@ mod tests {
         map.sync(&graph);
         assert_eq!(map[v], 5);
     }
+
+    #[test]
+    fn surviving_vertex_readable_after_delete() {
+        let mut graph = Graph::new();
+        let v1 = graph.add_vertex();
+        let v2 = graph.add_vertex();
+        let mut map = graph.vertex_map(0);
+        map[v1] = 1;
+        map[v2] = 2;
+        graph.delete_vertex(v2);
+        assert_eq!(map[v1], 1);
+    }
+
+    #[test]
+    fn capacity_does_not_shrink_after_delete() {
+        let mut graph = Graph::new();
+        let v1 = graph.add_vertex();
+        let v2 = graph.add_vertex();
+        let mut map = graph.vertex_map(0);
+        let capacity_before = graph.vertex_capacity();
+        graph.delete_vertex(v2);
+        map.sync(&graph);
+        assert_eq!(map.data.len(), capacity_before);
+        assert_eq!(map[v1], 0);
+    }
+
+    #[test]
+    fn reused_slot_returns_old_value() {
+        let mut graph = Graph::new();
+        graph.add_vertex();
+        let v1 = graph.add_vertex();
+        let mut map = graph.vertex_map(0);
+        map[v1] = 99;
+        graph.delete_vertex(v1);
+        let v2 = graph.add_vertex();
+        // VertexMap uses raw indices, not vertex identity. A new vertex v2 reusing the slot of
+        // previously deleted vertex v1 sees the old value. Callers must reinitialize stale
+        // slots after deletion.
+        assert_eq!(map[v2], 99);
+    }
 }