As a side effect, contruction queue can get stuck :

(I think the ruins have to be on a valid build tile to be able to do that)

- Add a "clear ruins" operation in the construction queue.

- Add a district construction near the ruins you planned to clear, without any adjacent district, city, ...

-> Once ruins are cleared, district construction cannot be completed, as target tile is now invalid.

Removing the invalid construction from the queue unlocks it.