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/llm_client.rs

@@ -743,4 +743,49 @@ mod tests {
         assert_eq!(store.nodes[1].parent_id, Some(parent));
         assert_eq!(store.nodes[1].id, child);
     }
+
+    #[test]
+    fn branch_options_handles_nested_forks() {
+        // Regression: verify sub-fork display when there are multiple forks.
+        // Structure:
+        //   root (user "q1")
+        //   ├── a1 (assistant "answer 1")
+        //   └── a2 (assistant "answer 2")
+        //       ├── q2 (user "q2")
+        //       │   ├── r1 (assistant "reply 1")
+        //       │   └── r2 (assistant "reply 2")  <- active leaf
+        //
+        // branch_options_at(root) should return [a1, a2] (2 branches)
+        // branch_options_at(q2) should return [r1, r2] (2 sub-branches)
+        let mut store = ChatHistoryStore::empty();
+        let root = store.append_node(None, mk_msg("user", "q1"));
+        let a1 = store.append_node(Some(root), mk_msg("assistant", "answer 1"));
+        let a2 = store.append_node(Some(root), mk_msg("assistant", "answer 2"));
+        let q2 = store.append_node(Some(a2), mk_msg("user", "q2"));
+        let r1 = store.append_node(Some(q2), mk_msg("assistant", "reply 1"));
+        let r2 = store.append_node(Some(q2), mk_msg("assistant", "reply 2"));
+        store.active_leaf_id = r2;
+
+        // Root fork: should show [a1, a2's subtree]
+        let opts = store.branch_options_at(root, r2);
+        assert_eq!(opts.len(), 2);
+        assert_eq!(opts[0].id, a1);
+        assert_eq!(opts[0].position, Some(1));
+        assert_eq!(opts[0].snippet, "answer 1");
+        // a2's subtree includes r2, so the rep should be r2 (preferred leaf)
+        assert_eq!(opts[1].id, r2);
+        assert_eq!(opts[1].position, Some(2));
+        assert_eq!(opts[1].snippet, "answer 2");
+
+        // Sub-fork at q2: should show [r1, r2]
+        let opts = store.branch_options_at(q2, r2);
+        assert_eq!(opts.len(), 2);
+        assert_eq!(opts[0].id, r1);
+        assert_eq!(opts[0].position, Some(1));
+        assert_eq!(opts[0].snippet, "reply 1");
+        assert_eq!(opts[1].id, r2);
+        assert_eq!(opts[1].position, Some(2));
+        assert_eq!(opts[1].snippet, "reply 2");
+    }
 }
+