Next-level cache should update after solving

[SimonN]

Lix 0.9.40

1. Have three unsolved singleplayer levels: A, B, and C, in this order, in direct succession.
2. From the singleplayer browser, play B.
3. Solve B.
4. Go back to the singleplayer level browser, play A.
5. Solve or exit A to go to the end-of-level screen.

Expected: End-of-level screen offers B as next level and C as next unsolved level.

Observed: End-of-level screen offers only B.

Speculation: The cache for checkmarks and the cache to find the next unsolved level are two different caches. The checkmarks are properly updated after solving B. But the next-level cache is not rebuilt, not even the node for B, after solving B. That cache, too, should be updated.

Maybe it's worth it to merge the two caches in the long term.