cc @SomeoneToIgnore

## Summary

Follow-up to #56030 

This mechanically extracts two editor topics into focused sibling
modules:

- `crates/editor/src/fold.rs`
- `crates/editor/src/selection.rs`

One odd boundary remains: several selection state types still live in
`editor.rs`. I didn't move them because those caused that "huge 11k
diff" in the previous PR, so I propose to move them later.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A