Replace public mutable collection fields/properties with private
backing fields and expose them as IReadOnlyList. Add proper methods
for mutation (AddItem, AddItems, RemoveItem, SortItems).
Changes:
- Bin.Items: Now private with AddItem/AddItems/RemoveItem/SortItems
- Result.ItemsNotUsed: Now readonly with Add methods
- Result.Bins: Now readonly with Add methods
- Updated all engine classes to use new encapsulated APIs
This improves encapsulation and prevents external code from
bypassing business logic by directly manipulating collections.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
