Show a diff / change preview before saving a segment edit

## Summary
Segment edits have no diff or change preview. Nothing surfaces what changed before an edit is saved, and there is no guard against an accidental rule deletion. A mistaken change can ship silently.

## Requested improvement
Show a diff or change preview for a segment edit before it is applied, so the user can confirm what is being added, changed, or removed.

## Related issues
- #6726 Show segment rule changes (diff) in audit log detail view

No files, tests, or entry points are named. Start by locating the segment-edit form and its save flow, then inspect how segment rules are represented. Done means users can review additions, changes, and removals before saving, with accidental rule deletion clearly surfaced for confirmation.

Written by the indexing model from the issue text.