CanvasView:
- Fix Save button visibility - now shows when spec is loaded (grayed if no changes)
- Separate logic for spec mode vs legacy mode save buttons
- Fix Reload button visibility
IntrospectionPanel:
- Add Mass Properties section (mass, volume, surface area, CoG, body count)
- Add Linked Parts section showing file dependencies
- Add Bodies section (solid/sheet body counts)
- Add Units section showing unit system
- Type-safe access to all nested properties