fix(barrel): preserve data across world reloads

[Minecraft0122]

Summary

• register the existing world load/unload listener
• reload persisted barrels when a world becomes available
• save and unregister barrels when their world unloads
• preserve unloaded-world barrel rows during autosaves
• explicitly delete persisted rows when a barrel is actually destroyed
• preserve Folia barrel data when detecting an unloaded world without an unload event

This replaces the debug-only approach in #198 with the actual fix.

Fixes #197.

Testing

• ./gradlew compileJava --console=plain --no-daemon