Structurally identical polymorphic types that only differ by typvar names don't have join

<img width="1228" height="305" alt="Image" src="https://github.com/user-attachments/assets/946d9c84-a6cc-442b-a029-bffee27c5172" />

<img width="1050" height="39" alt="Image" src="https://github.com/user-attachments/assets/c75a874a-bf6a-45d5-b196-f4d0616d877b" />

Expected behavior: these two types are the same