Layout: Handle orphaned display rules

[JeroenDeDauw]

Follows-up to #692

When a Schema property is renamed or deleted, Layout display rules referencing it become orphaned. Currently these are silently filtered out in the editor and dropped on save.

Options:

• Current behavior
• Show orphaned rules with a visual warning in the editor
• Auto-remove them on editor open with a notification
• Preserve them as-is and only warn

Context: the NeoWiki codebase and architecture review of PR #692.
_{Written by Claude Code, Opus 4.6}