Saved-world mod mismatches

Hooks::mod_mismatch reports changed and removed saved mods whenever the world's saved list exists, including empty and unchanged lists; newly added mods and minecraft are excluded. Both logical sides can open worlds, and singleplayer opens once on the integrated server.

Unlike NeoForge's raw-read timing, this runs at the earliest available Host before world-state restoration, after preparation, because it only reports and logs and its resolution output does not depend on the earlier moment.

Versions use the loader's semantic version comparison, padding omitted minor and patch components with zero; invalid or empty versions compare as exact text. Mark a migration with event.resolved.push("my_mod".to_owned().into()). The first listener to mark an id owns its resolver attribution; later listeners cannot erase or replace that marker. Markers for other ids are allowed. Differences are logged sorted by mod id, resolved at debug with the resolver and self/other distinction, unresolved at warn. No dialog or cancellation is added. The current mod list is written on the next save, not when this event fires.