Add ElementMap::reserve to replace ElementMap::expand, update tests

ElementMap tests are now self contained unit tests, without any graph context. The only exception is the repurposed test "reused_slot_returns_old_value", which is not suited for GraphTopologyDeletion, but actually specific for Graph.

src/models/graph.rs

@@ -420,4 +420,27 @@ mod tests {
         graph.delete_edge(f);
         assert_eq!(graph.edge_count(), 0, "unexpected edge count after delete");
     }
+
+    #[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();
+        assert_eq!(
+            v1.arr_idx(),
+            v2.arr_idx(),
+            "precondition: new vertex {v1:?} should reuse slot of deleted vertex {v2:?}"
+        );
+        // ElementMap uses raw indices, not vertex identity. A new vertex v2 reusing the slot
+        // of previously deleted v1 sees the old value. Callers must reinitialize stale slots
+        // after deletion.
+        assert_eq!(
+            map[v2], 99,
+            "new vertex reusing slot of deleted vertex should return old value"
+        );
+    }
 }