test: add nested fork regression tests

- branch_options_handles_nested_forks: verifies branch picker returns
  correct branches for both root fork and sub-fork
- render_tree_path_nested_forks_only_emits_at_divergence: confirms fork_info
  doesn't propagate downstream (only at actual divergence point)

src/ai/insight_chat.rs

@@ -2894,6 +2894,37 @@ mod tests {
         assert_eq!(f.node_id, u, "divergence node is the forked user turn");
     }
 
+    #[test]
+    fn render_tree_path_nested_forks_only_emits_at_divergence() {
+        // Regression: with .take() instead of .clone(), fork_info should only
+        // appear at the actual divergence point, not propagate downstream.
+        // Structure:
+        //   u (user "q1")
+        //   ├── a1 (assistant "answer 1")
+        //   └── a2 (assistant "answer 2")
+        //       ├── q2 (user "q2")
+        //       └── r2 (assistant "reply 2")  <- active leaf
+        //
+        // Expected fork_info: [None, Some, None, None]
+        // (only a2 has fork info since it's the divergence from a1)
+        let mut store = ChatHistoryStore::empty();
+        let u = store.append_node(None, ChatMessage::user("q1"));
+        let _a1 = store.append_node(Some(u), assistant_text("answer 1"));
+        let a2 = store.append_node(Some(u), assistant_text("answer 2"));
+        let q2 = store.append_node(Some(a2), ChatMessage::user("q2"));
+        let r2 = store.append_node(Some(q2), assistant_text("reply 2"));
+        store.active_leaf_id = r2;
+
+        let path = store.path_to_leaf(r2).expect("path to active leaf");
+        let (rendered, _turns, _node_ids, fork_info) = render_tree_path(&store, &path);
+
+        assert_eq!(rendered.len(), 4, "user + asst + user + asst");
+        assert!(fork_info[0].is_none(), "initial user has no fork");
+        assert!(fork_info[1].is_some(), "a2 is the fork point");
+        assert!(fork_info[2].is_none(), "q2 should NOT carry fork forward");
+        assert!(fork_info[3].is_none(), "r2 should NOT carry fork forward");
+    }
+
     #[test]
     fn rewind_strips_assistant_and_tool_scaffolding() {
         // Rendered: [user1, asst1, user2, asst2] → cut at rendered index 3