Implements ICardInventoryRepository.SetProtected — loads only the
owned-cards collection (no decks/currency), guards against the EF
owned-nav Card.Id==0 default-init quirk, and accepts Count=0 rows
(destruct→re-protect round-trip). Covered by 4 new NUnit tests
(flip, unset, zero-count-row, unknown-card error). Full suite: 533/533.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
