Worldsure

First public version, for Minecraft 26.3, 26.2, 26.1.x, 1.21.11 and 1.21.1 on Fabric and NeoForge, and 1.20.1 on Fabric and Forge.

• Every world keeps a passport of the mods it was played with (<world>/worldsure/manifest.json).
• In single player, opening a world whose mods changed first shows the Worldsure screen: every removed, downgraded, updated and added mod as a card with how many blocks, chests or machines and mobs of it the world holds, an "All mods" tab, and "Back up & play", "Play without backup" and "Back". The same screen opens from a button in the pause menu, showing the backup that was made.
• On a dedicated server the world is backed up to backups/worldsure/ before any chunk loads. The 5 newest backups per world are kept (config/worldsure.properties).
• Blocks of a removed mod are kept: while the mod is missing they show as an unbreakable "Block of a removed mod", and the original blocks and block entities are written back when the chunk saves. Reinstalling the mod restores them.
• Counts of blocks, block entities and entities per mod are kept in <world>/worldsure/index.dat.

Known limits: entities and items of a removed mod are still dropped by the game, and counts only cover chunks saved while Worldsure was installed. Worldsure adds a block, so it must be installed on both the server and the clients.